So here we go – another questionably qualified celebrity woman talking about her pain. I'm rolling my eyes as much as you are, I promise. But I told myself that one day I'd write this, and my word is my bond, and I'm the person who has to look at me in the mirror every day. I don't always like what I see, but maybe writing this will change that a little. Maybe it won't. Maybe I'll delete it. But if you're reading this then I must've plucked up the courage or lunacy or self-indulgence to send it out into the world, and whether or not you keep reading doesn't really matter anymore, not to me. Because I'll already have done the hard part.

I was told recently that it is Endometriosis Awareness Month. The person who told me this was my sister-in-law, who has known me since I was ten years old, and is one of only two people (besides my doctors) who knows about my health enough to think that telling me it's Endometriosis Awareness Month is important.

See, Anne has known me since 'before.' It's hard to even think of a before and after to your womanhood, because once it starts you don't really remember what it was like to not bleed every four weeks. It becomes a slow steady drumbeat and rhythm to your life, a method of telling time, in a way. The first few times are hard – easier when you have a calmly no-nonsense mum like mine, who isn't fazed by what her 11-year-old thinks is 'gross,' and a sister-in-law to take you out for ice cream and tell you that it's normal and that being a woman is special, and that this can be your little secret, that she won't tell your brother, because it's between 'us girls.'

And it is. Being a woman is special. But being a woman also fucking sucks.

I, like so many other girls, was told that discomfort and pain every month was normal, that it was just part of the cycle, and that there were easy ways to combat it. And for a while, there were; low doses of ibuprofen a few days every month, usually one uncomfortable night spent on the couch with a hot water bottle, on and on it goes. I was stubborn even then, and would wait until I was particularly tender before giving in to intervention – no, Mum, I'm fine, I just need to lie down. Even knowing it was 'normal' wasn't good enough for me, because I wanted to be invincible. I was certain I was.

I don't remember when it went from discomfort to pain – maybe it always was. I do remember being sent to the infirmary after a field hockey practice when I was sixteen, because I'd been limping. I wouldn't let the nurse touch me, and said I'd only twisted my ankle; I insisted I'd dealt with something like that before when she suggested bringing me to hospital, and that sending me back to my dormitory with ice and paracetamol would be fine. Meanwhile, sitting calmly on her examination table, my insides were practically pulsing; the 'limp' was a reflex to protect my abdomen.

It would be more than a decade before a doctor told me that the pain I felt was far beyond the normal scope of menstrual discomfort. I was doing a play in London, and some days I was in so much pain that I could hardly stand up straight to get on stage. I would take double the recommended dose of ibuprofen, lie down until the very last moment before places, and collapse in my dressing room at the interval. My director was (understandably and logically) worried about me; I'm the sort to run myself

In a rare moment of honesty with myself, I engaged a private gynaecologist; this was not a cold sending me to a GP, I knew. I'd always tempered my disclosure of menstrual pain to doctors, especially as I got older and developed some odd form of pride in having a high pain tolerance from years of dealing with it. And it had come and gone throughout my life, so there was pride, too, in my surety that it would subside again. But this time I was honest. I consider myself a truthful person, but it was hard to acknowledge how much I had been hurting. I felt consumed by the pain radiating from inside me, not least because I felt hideously alone.

There were scans and much poking and prodding. A few weeks later, he had settled on a diagnosis of endometriosis, and prescribed me your average hormonal birth control pill. I had been on and off it throughout my twenties, as many women do when relationships come and go; I'd been blind to the pattern that my pain lessened during those times, perhaps because I thought companionship was what made me happier, not less suffering every month. I felt like a failure for not having investigated the pain sooner, knowing that there was so easy a way to temper it – while it did not eliminate my pain entirely, the pill did diminish it, as it can do for many women with endometriosis, provided their bodies respond to it and they don't experience other side effects.

I have since learned that it's sort of miraculous that my doctor came to his diagnosis so quickly and accurately – many women suffer for years, and are told their pain is everything but what it is. Apparently even in 2011 (or now in 2017), there's a tendency to dismiss gynaecological pain symptoms because it's 'normal' to have cramps (a word that makes them sound hilariously benign), etc. I think it's ridiculous that we have to say this at all, but it also makes me grateful for a doctor who is highly professional, compassionate, and knows when I am not bullshitting him.

I stayed on the pill for about two years. I still had some pain every month, but it was considerably less than before. However, I could tell something was different about my body. I just knew. I didn't feel 100% myself physically and sexually, and that would have been fine for most people. It should have been fine for me.

But with the summer of 2013 came the role of my lifetime, and with her a conviction that I had to feel everything it meant to be a woman, pain and all. Against the advice of my doctor, and without the knowledge of my trainer and director (to whom I had not disclosed my medical condition), I discontinued the medication.

For a while, I was great – better than great. I'd taken up an intense training regimen, and I was firing on all cylinders. Adrenalin really does dull pain, and my life was full of it. On days off I was boxing or going through circuits for hours, going for the best-paced runs I had in years. There was an undeniable and immense satisfaction in having passed thirty and yet being in the best shape of my life, with the greatest professional challenge of my life.

Once again, I felt invincible. But I wasn't.

After several months in Missouri and Los Angeles, I moved on to Toronto for my next project. It was winter there, and I was suddenly cooped up for hours learning piano, and walking around on a vast set in heavy dresses and wigs. As quickly as the adrenalin had come, it left me. The rollercoaster of gaining weight quickly by bingeing on burgers and milkshakes only to drop it just as fast had caught up to me, and my stomach was rebelling. So, too, was my fucking uterus.

I've shared this anecdote before, of a take during this film when I ran down a corridor at breakneck speed in a pair of seven-inch heels and a diaphonous nightgown, only to completely trip myself up and crash into the floor and wall. I laughed it off, because I wasn't hurt, much to everyone's relief. But in that moment, when I was sprawled out on the floor, a heavy brown wig tangled around my body, I almost couldn't get up. It had taken nearly all of my strength to even stand and leave my trailer that morning without doubling over with the pain inside me, and I wasn't sure I could do it again.

But I did it. I got up. I shook it off, said I was fine.

When I got home to London, I had my doctor prescribe me a very small amount of narcotics, for emergencies only. I didn't want to ask, but looking helpless in front of him was somehow better than having to sit down at work to catch my breath on a bad pain day. He strongly advised me to go back on the pill, but now, suddenly bitter at my thirties when only six months earlier I'd felt in command of them, I declined. I wasn't in a relationship, and he had already informed me that fertility could be an issue given my condition. Why even bother?

I'm not saying this to make myself look like an idiot (though I invite you to think I am, because you're probably not wrong), or to clarify that my gynaecologist is neither incompetent nor irresponsible (though of course he's both highly competent and responsible, not to mention patient and far smarter than I am), and certainly not to make some point about how women are conditioned to lie about the pain they're in – because God knows I don't want to leverage my very personal, private experiences into some social argument, or else I would have said something before now.

What I am trying to say, more than anything, is that sometimes the truth can set you free, and it doesn't need to be more than that. I'm hoping it does, or all of this writing will have been for nothing.

I turned 35 this year. Most doctors will refer to this as 'advanced maternal age' if you start discussing wanting children. While endometriosis is still being studied and has varying effects on the women (an estimated 10% of the female population of childbearing age) diagnosed with it, it's considered a risk to fertility, especially if, like me, you've had excision surgery. It is part and parcel of being a 35-year-old woman (or frankly, a woman with a life of her own who seems vaguely stable) to be asked if you want a family – and again, I'm not about to get on some fucking tangent about this being hideously antiquated and chauvinistic social discourse in which we see a woman's value first and foremost as a bearer of children, but it's nonetheless the way things are. I accept that.

This is part of why I never disclosed my medical condition to my parents. Other people I can handle – if I'm good at anything, it's brushing off questions I don't feel like answering without making the one who queried feel too bad for it – but telling one's parents that you have been in pain right under their nose and may never have children of your own is a big fucking burden. I love my parents, but I want to save them from taking my suffering on as theirs.

But it eats at me. It makes me sick, sicker than I am anyway. I'm such a stubborn person that I'm even mad at myself for disclosing my condition to my brother, only because I needed someone to take me to hospital. He's my older brother, my closest friend, and I hadn't told him for twenty-five years that I was suffering.

Which brings me back to my sister-in-law, and motherfucking Endometriosis Awareness Month. I realise I've saddled my brother and his wife with their own sort of burden, by asking them to keep my medical condition to themselves, and also rather vehemently not accepting their advice or concern. I owe them an apology for that – one day I'll give it, or I suppose have the balls to make this public somewhere and want to tell them my feelings beforehand.

But what most bothers me is not the pressure to disclose my pain for my own good, but the tone that in some way I 'must' share. 'You could help people, make them feel less alone,' Anne has told me. And she's not wrong. But still – why? A 'month' for it? I feel it every fucking day.

Chronic pain is personal and isolating and scary, and everybody handles it differently. For me, it is intensely private. It is something I live with moment to moment, like an old enemy who I've kept as too close a friend, who may take from me the one thing I want more than anything else in the world. And I don't want to fucking share it. It hurts too badly. Even obliquely talking around it hurts.

There are the Lena Dunhams of the world who share their most intimate of struggles as a means to help others, and make some sort of statement about how we should be honest about our pain and not be cowed into silence because it makes people uncomfortable, and not ourselves be uncomfortable because we're taught by society that our pain doesn't matter or is embarrassing. That's their right. But fuck, I'm uncomfortable because I'm sad. I'm uncomfortable because I hurt most days and just don't want to share. And some misguided expectation that a person like me should share things strictly to help others forgets one person – me. It isn't selfish to put on your own oxygen mask first.

I try not to listen to the chatter, but I get hopelessly angry sometimes. Angry at myself, for being so stubborn with my health and who knows about it, for even wanting children when it's not a given, for going to work on strong painkillers a day or two each month. Angry at the world, for implying someone like me is selfish for keeping silent, for not accepting a moment's finite discomfort that I can supposedly afford in order to make a bigger statement with the platform I have.

I don't want to suffer more than I have to. I just don't. Which is why I'm not even sure why I wrote this, besides to make sense of it for myself. Which you're probably going to see as selfish. But that's okay. When you hurt every day, sometimes you have to only look out for #1. That should be okay.|