Everyone has a tipping point. The funny thing is that when mine came, in March 2012, it wasn't something dramatic. It was just another week of little pinpricks: the man who appeared as I sat outside a cafe, seized my hand and refused to let go; the guy who followed me off the bus and propositioned me all the way to my front door; the man who made a sexual gesture and shouted, "I'm looking for a wife" from his car as I walked home after a long day. I shouted back, "Keep looking!" but as I trudged home, I started for the first time really to think about how many of these little incidents I was putting up with from day to day.

I thought about the night a group of teenage boys had casually walked up behind me in the street before one of them grabbed me, hard, between the legs. I recalled the boss who'd sent me emails about his sexual fantasies and terminated my freelance contract with no explanation almost immediately after learning I had a boyfriend. I remembered the men who cornered me late one night in a Cambridge street, shouting obscenities, and left me cowering against the wall as they strolled away.

And the more these incidents came back to me, the more I wondered why I'd played them down at the time – why I'd never complained. The answer was that these events hadn't seemed exceptional enough for me to object to. Because this kind of thing was just part of life – or, rather, part of being a woman. And I started to wonder how many other women had had similar experiences. So I started asking around. To my surprise, every woman I spoke to had a story. And they weren't random one-off events, but reams of tiny pinpricks – like my own experiences – so niggling and normalised that to protest about each one felt trivial. Yet put them together, and the picture was strikingly clear. This inequality, this pattern of casual intrusion whereby women could be leered at, touched, harassed and abused, was sexism. And if sexism means treating people differently or discriminating against them purely because of their sex, then women were experiencing it on a near-daily basis.

The more stories I heard, the more I tried to talk about the problem. And yet time and again I found myself coming up against the same response: "Sexism doesn't exist any more," people told me. "You're uptight, or frigid… you really need to learn to take a compliment."

People didn't want to acknowledge it, or talk about it. And it wasn't just men who took this view; it was women, too, telling me I was being oversensitive, or simply looking for problems where there weren't any.

At first, I wondered if they were right. I thought I'd take a look at the statistics. I found that in this supposedly "equal" society, with nothing left for women to want or fight for, they hold less than a quarter of seats in parliament, and only four out of 22 Cabinet positions. That just seven out of 38 Lord Justices of Appeal and 18 out of 108 High Court judges are female. That in 2010 it was reported that the National Gallery's collection of some 2,300 works contained paintings by only 10 women. That our Royal Society has never had a female president and just 5% of the current fellowship is made up of women. That women write only a fifth of front-page newspaper articles. That women directed just 5% of the 250 major films of 2011, down by nearly half from a paltry 9% in 1998. I found that on average more than two women are killed every week by a current or former partner, that there is a call to the police every minute about domestic violence, and that a woman is raped every six minutes – adding up to more than 85,000 rapes and 400,000 sexual assaults per year.

I didn't for a moment think that the problem of sexism could be solved overnight. But nor did I see how we could even begin to tackle it while so many people continued to refuse to acknowledge that it existed. So, in April 2012, I started a simple website, everydaysexism.com, where women could upload their stories. Without any funding, or means to publicise the project beyond my own Facebook wall, I thought perhaps 50 or 60 women would add their stories. Stories began to trickle in during the first few days. Within a week, hundreds of women had added their voices. I started a Twitter account, @EverydaySexism, and found that people were keen to discuss the phenomenon there, too. Stories began to appear from America and Canada, Germany and France, Saudi Arabia and Pakistan. Tens of thousands of people started viewing the website each month. Within 18 months we had expanded to 18 countries. In December 2013 – 20 months after the project was launched – we had 50,000 entries.

I have been asked what has shocked me the most since starting the project. I think people expect me to say that it's the stories of rape or violence. Those stories angered and devastated me, of course, but nothing has shocked me more than the thousands of entries from girls under the age of 18.

One day, in the early months of the project, I read two or three entries in a week from girls who had been subjected to leering and shouting from men in the street while walking home from school in their uniform. Dismayed, I posted a question on Twitter: surely this couldn't be a common occurrence? By the end of the day, a deluge of tweets had confirmed that the experience was not only common, but almost ubiquitous. One post read: "I have had men say things like 'Hello darling' and 'Sexy lady' while walking home from school in my uniform."

The obsessive focus on girls' looks is particularly poisonous. One girl noted her bemusement at having her legs commented on, aged just 10: "I'd never thought much about my legs before, they were just something I walked on." As girls hit the age of 10 or 11, this obsession with their appearance takes a distinctly sexualised turn. Suddenly they are defined not only by their looks, but also, more specifically, by what boys and men think of them.

This often translates into a single, all-encompassing quest for thinness. A 2012 report from the All-Party Parliamentary Group (APPG) on Body Image revealed that girls as young as five are worrying about their size and looks, and that one in four seven-year-old girls has tried to lose weight. The Representation Project, a US-based campaign working to reveal gender inequality and shift public perceptions, has revealed that the number one "magic wish" for young girls aged 11 to 17 is to be thinner. It takes a while for that last figure to sink in. Just think of all the other things in the world that teenage girls could wish for.

One girl, aged 14, tells me that girls worry about their weight "all the time": "The girls on the internet are perfect, and the girls know the guys talk about it and I guess they want to please the guy. If you don't have a thigh gap, you need to get a thigh gap." (For those not in the know, a "thigh gap" is achieved when a woman stands with her legs straight and together, and there is a gap between her thighs.) When I ask how the girls go about this quest for "perfection", she confidently explains: "Year 7 or 8, it's really common not eating, but when you get to 14 to 18, it's more diet pills and exercising all the time."

Another girl, a sixth former, meets me in a quiet north London cafe. "I don't think I know any girls who don't have some sort of self-esteem or body-image issues," she tells me. "I have about six friends with eating disorders and so many friends who don't have eating disorders but they're disordered eaters – they'll eat only fruit for a day or something. And so many people who just feel anxious about it – they won't wear short-sleeved shirts because they're embarrassed about their arms." I ask her how far back the problem goes, and her eyes glaze as she recalls a childhood of intense self-scrutiny: "I remember being embarrassed about my thighs aged six. I remember girls comparing their bodies in the toilet in Year 5. I would have been about nine."

She is beautiful, this 17-year-old girl who tells me: "I feel like people are watching me all the time, judging me. I never show skin or anything. I feel way too self-conscious to do that."

I have never felt as angry or as frustrated as I did that afternoon, wishing there was anything in the world I could say to make this teenage girl realise how much she had to be proud of, and knowing that nothing I could say would change the way the world had made her feel about herself.

She attributes her own slide into the grip of an eating disorder, aged 15, to the increasing levels of street harassment she was experiencing at the time: "There was a man who ran his hand up my leg on the tube – that was one of the first times I came to London to stay with my sister. I felt embarrassed and scared… I didn't want to make a fuss. Losing weight seemed like the appropriate reaction to being looked at as a sexual being. It's kind of reclaiming your body, but in a really negative way. I remember feeling completely powerless."

Every one of the girls I've spoken to described sexual harassment as a regular part of life. According to a 2010 survey by YouGov for the End Violence Against Women Coalition, nearly one in three 16- to 18-year-old girls said they had experienced unwanted sexual touching at school. A huge 71% of all 16- to 18-year-olds said they heard sexual name-calling (such as "slag" or "slut") towards girls at least several times a week. And yet it seems girls are being given neither the resources to deal with it, nor the information to understand that they shouldn't have to face it in the first place.

They also then get ripped to shreds for being too sexy. Enormous pressure is put on very young girls to be sexually active, to give in to boys' "demands" and to acquiesce to various requests. But the moment they comply, they face a stringent backlash.

One 14-year-old girl says: "If you're talking to a guy or you text with him, he will ask for a picture." Photograph: Getty Images

One 14-year-old girl tells me, "If you're talking to a guy or you text with him, he will ask for a picture." I ask why girls feel they have to comply. "You'd feel like you don't want to let him down – you think he likes you for who you are and he promises not to show it to anyone, then you send the picture and then he'll never speak to you again. The guy shows his friends and then the friend puts it up on the internet and then for the girls it's horrible – her friends will turn against her and call her a slut, and the guys at school will all come up and say they saw the picture and she'll lose all her friends." As extreme as it sounds, versions of this story are relayed to me again and again, by girls from all backgrounds.

What makes the cycle of pressure and judgment even more powerful is that, thanks to social media, there is no escape from it, even at home. With this absolute internet focus comes instant, easily accessible porn. In a group interview, one sixth-form girl tells me: "The view of women through porn creates assumptions – it means [boys] just expect women will take it, the man's in control; and I don't think they can separate that the woman is acting and that isn't what relationships are really like… most of the boys will probably have been watching it since about 14 – that's how they learn about sex." Another 17-year-old girl agrees: "Boys in my school were watching porn in Year 7, possibly earlier. They started circulating pictures. And they were also making rape jokes – like saying, 'You're so hot, I'd rape you.' "

In a heartbreaking Everyday Sexism Project entry, one schoolgirl wrote: "I am 13 and I am so scared to have sex it makes me cry nearly every day. We had sex education in Year 6 and I felt fine about it, but now some of the boys at school keep sending us these videos of sex which are much worse than what we learned about and it looks so horrible and like it hurts, and at night I get really scared that one day I will have to do it."

Nothing has emerged more clearly from the Everyday Sexism Project than the urgent need for far more comprehensive mandatory sex-and-relationships education in schools, to include issues such as consent and respect, domestic violence and rape. It's not just girls who need it. For boys, porn provides some very scary, dictatorial lessons about how they are expected to exert their dominance over women. It is unrealistic to expect them, unaided, to work out the difference between online porn and real, caring intimacy.

When we carried out an online poll, asking people whether their school sex-and-relationships education had covered issues such as domestic violence, assault or rape, more than 92% said these issues were never raised at all. These statistics were borne out by numerous entries from girls and young women feeling confused and anxious about sex and consent. Huge numbers simply had no idea they had the right to say no.

Young people need to be aware of the possibility that things could be different. It isn't sweeping reform or mass changes of the law that we need now, but a change in ideas and inherited assumptions about rape, about body image and vulnerability. And that is something everybody can contribute to. Enough is enough.

The picture might seem depressing and at times I have felt overwhelmed. But setting up Everyday Sexism has also put me in touch with a number of inspiring young people who are passionate about changing things for their generation. Despite facing a barrage of pressures, they display tenacity, enthusiasm and courage in the battle for gender equality. They are our great hope..

• This is an edited extract from Everyday Sexism, by Laura Bates published next month by Simon & Schuster at £14.99. To order a copy for £11.99, including free UK p&p, call 0330 333 6846 or go to theguardian.com/bookshop.

Read more by Laura Bates in Tuesday's G2, when she will also take part in an online Q&A at theguardian.com.