In this exclusive extract from her new book, the feminist activist talks about her eating disorder, the pressure on girls to be beautiful and why weight is a political issue

Aged 17, I am something of an anomaly when I arrive on an eating disorders ward. Close-cropped hair, black clothes, soaked in hair dye and riot grrrl rock, dressed as a boy, obviously queer. It is only later that I will learn that between a quarter and a half of young people hospitalised with eating disorders are gay, trans or genderqueer. That's one of the things they don't tell you about how and why young girls fall apart.

The young women already there look like broken dress-up dolls, all of us poured from the same weird, emaciated mould, barely able to stand upright, the same cut marks scored like barcodes in the secret places on our skin. Clearly, the other girls have starved themselves to the point of collapse simply because they want to look pretty; I, meanwhile, have perfectly rational, intellectual reasons for doing exactly the same. We will never be friends. We have nothing in common.

This point of view lasts almost exactly 18 hours, until the first scheduled late-night feeding time, when we all huddle together on cheap hospital sofas trying to push two puny biscuits into our faces, feeling boiled in our skin. One girl, who is 10 years older than me and has her own story, shunts close and puts a bony arm around my shoulders. "It's all right," she tells me. "You can do it."

I allow myself to be held. I pick up the biscuit. And something changes.

Over the weeks and months of confinement, these girls will become my greatest friends. I will learn at 17 what it takes some people decades to accept: that pretty girls who play to patriarchy and ugly girls who never got asked to a school dance suffer just the same. That the same trick is being played on all of us. There is no way to play the perfect-girl game and win.

The cruellest lie they were told was: "It's what's on the inside that counts." It is not what's on the inside that counts. Perfect girls don't get a day off. Perfect girls don't sit on the sofa eating biscuits, even when their very favourite show is on. Perfect girls are always working; when they are not at school or on the clock they are working out, and when they aren't working out they are volunteering, shopping or running a social life like a frantic startup. Be a good girl. Smile and make people feel comfortable; accept low pay, long hours, the occasional grope in the corridor, compete with the other young women to be the prettiest and most compliant, or the hardest-working, or the girl everyone loves. Just don't ever aspire to be more than that.

Being That Girl is easy if you're white and averagely pretty. There is no trick to it. You don't even have to totally excise the parts of your personality that don't fit, the parts that are smart and difficult and loud and angry and ambitious and masculine and mature. You just dial those parts down until they become background noise; dial them down and down until the male ear can't pick up their frequency and pretty soon you can't even hear them inside your own head. Tune them out and swallow them down like the hot meals you don't eat any more because That Girl must stay slim and fragile if she wants to be beautiful and loved. And you do want to be beautiful and loved.

Eating disorders are easier to conceal than most mental illnesses, especially in a visual culture where we've got used to images of extremely undernourished young people. Those that do not necessarily cause extreme weight loss, such as bulimia nervosa and compulsive overeating disorder, are easier still to keep secret – for a while. All these illnesses take a frightening toll on the brain and body, both in the long and short term, as sufferers turn to all kinds of dangerous and grotesque methods to control their weight, from bloodletting, drug abuse and frantic overexercise to vomiting until the sufferer's cheeks swell and teeth rot from spewing stomach acid.

It's not pretty. It's the ugly little secret behind much modern beauty culture, and the biggest secret is that it's no secret at all. None of it is. Diagnosis of eating disorders, chronic self-harm and other, more arcane forms of self-injury has mushroomed over the past decade, especially among girls, young queers, anybody who is under extra pressure to fit in.

Of all the female sins, hunger is the least forgivable; hunger for anything, for food, sex, power, education, even love. If we have desires, we are expected to conceal them, to control them, to keep ourselves in check. We are supposed to be objects of desire, not desiring beings. We do not need food: in many ways, we are food, trainable meat, lambs queueing up to buy mint sauce. We consume only what we are told to, from lipstick to life insurance, and only what will make us more consumable ourselves, the better to be chewed up and swallowed by a machine that wants our work, our money, our sexuality broken down into bite-sized chunks.

Men experience body policing too, of course, and there are real penalties for being overweight. The penalties, however, tend to be less existential; one can still, outside a very small range of professions, expect to be judged as a man first and as a body second.

It is interesting that "ugly" is still the insult most commonly thrown at women to dismiss their power, to get them to shut up. Female politicians are called ugly and unfuckable by men who can't quite bring themselves to say directly that they don't deserve their power, that their primary purpose as women should be to please and arouse the opposite sex. "Fat" is even more obvious. You're gross, you take up too much space, get out of my sight.

'We've got used to images of extremely undernourished young people.' Photograph: Timothy A Clary/AFP/Getty Images

The latest right-on theories about eating disorders posit them as a method that young women use to escape the stresses of modern femininity. Anorexia nervosa, the logic goes, suspends the traumatic process of becoming a woman, because when you stop eating, when you cut down from 600 to 400 to 200 calories a day, your periods stop, your tits and hips and wobbly bits disappear, and you return to an artificial prepubescent state, complete with mood swings, weird musical obsessions and the overpowering impulse to shoplift scrunchies from Woolworths. The reason young women and increasing numbers of young men behave like this, the logic goes, is because they're scared and angry about the gender roles they are being forced into. The notion that they might have damn good reasons for being scared and angry has not yet occurred to the psychiatric profession.

The more powerful women become, the more we are taught that our bodies are unacceptable. Many of the most influential women in the world, from pop stars to media tycoons, have faced public battles with their weight that the tabloid press is only too happy to catalogue and exaggerate. Others, particularly politicians, have faced popular ridicule for the apparently scandalous surfeit of flesh on their perfectly normal-sized bellies and bottoms.

A report published in a recent issue of the Journal of Applied Psychology revealed that the pay and influence of test groups of women in the US and Germany consistently rose as their weight dropped below the healthy average. By contrast, weight gain was an indicator of financial success for males up to the point of extreme obesity, when men too begin to pay a professional penalty.

Causality is always difficult to establish. Even with a study as rigorous as this, it is impossible to say conclusively whether the women lost weight because their salaries rose, or whether their salaries rose because they lost weight. One thing's for certain, though: in Europe and the US, fear of female flesh is fear of female power, and western society's stage-managed loathing for women's normal-sized bodies is deeply political.

That's nothing, however, when compared with the utter horror society reserves for larger women who are also poor. In western countries, where quantity of food access isn't as problematic as quality, being overweight is as likely to be a symptom of poverty and malnutrition. This fact has only cemented the barely concealed disgust of the cultural right for working-class women who take up too much space.

From boardrooms to the streets, women's anxiety to keep our body mass as low as possible is based on legitimate fears that we will be punished if we attempt fully to enter patriarchal space. No wonder so many of us are starving.

Society understands that young girls are fucked up. That's part of their charm. Not only are they objects, they are abject, terminally unable to cope with the exigencies of adult life, of the bewildering array of life choices modern society offers us, from vaginal butchery to jobs in the service sector.

Western womankind is collectively imagined as a toddler let loose in a candy store, so overwhelmed by the range of options that it has an ungrateful tantrum and is sick on the floor. And fucked-up young girls grow up to be miserable women: study after vaunted study tells us that women and girls are as miserable as they have ever been, overworked, exhausted, taking prescription medication in three times the numbers of men.

The front pages of celebrity magazines shriek out a chorus of successful women on the verge of mental and physical collapse: this star is starving herself, this one is depressed, this one drinking herself into a nightly stupor until her children are confiscated. It is a myth that pleases the powerful. Women have all this equality and opportunity now, but we can't handle it. Maybe we weren't meant to have it in the first place.

Perfect girls know that they must constantly improve. Of course, nobody is really a perfect girl.

You reach a point where you have to decide what you will sacrifice to survive. It was years ago now, and enough has happened to me since that I've forgotten when it was that I decided to give living a shot, just as an experiment, to see if I could. Maybe it was shuffling to the small medical kitchen to eat toast, for the first time, without fighting. I just remember the crisp, buttery bread, and the fear that if I let my hunger loose I'd never stop eating, I'd eat and eat until I was the size of a monster truck and keep eating until I'd swallowed the world. A young girl's hunger is a fearful thing.

Or maybe it was months later, leaving hospital for the first time in a new dress and lipstick I'd put on to convince the ward nurse that I was finally healthy, ready to live a healthy life, painting on an expression the way women learn to do when we have to convince the world we're happy. Waving goodbye to the friends I'd made there from the window of a taxi taking me hell knows where, only that I would not be going home ever again.

Being a good girl, a perfect girl, can kill you fast or it can kill you slow, flattening everything precious inside you, the best dreams of your one life, into drab homogeneity. At 17 I decided to make a stab at a different kind of life, and it was scary, and too much, and it still is, but so is staying at home with a painted-on smile. I see women making that choice every day, in their teens and 20s and 60s and 70s, and in this brave new world where empowerment means expensive shoes and the choice to bend over for your boss, it is the only choice that really matters.

Those who make it get called selfish bitches, freaks and sluts and cunts and whores, and sometimes we get called rebels and degenerates and troublemakers, and sometimes we are known to the police. And sometimes, in the narrow hours of the night, we call ourselves feminists.

• Unspeakable Things by Laurie Penny is out on 3 July (Bloomsbury, £12.99). To order a copy for £9.99 with free UK p&p, go to theguardian.com/bookshop or call 0330 333 6846.