It was a beautiful, balmy summer's evening the night Stewart O'Callaghan lost everything. At 10.30pm on 20 July, 2013, he and his partner Matt left a close friend's house and started walking home down Hackney Road in London's East End. The trees were in full bloom. The heat was just beginning to dip, a breeze picking up as their arms collided and hands linked. A few streets away, the music festival Lovebox boomed and throbbed, throngs of partygoers spilling out into surrounding roads.

For the first time, aged 26, Stewart was happy. After a childhood in Reigate, Surrey, with no money, no father, his mother on benefits and endless bullying for being gay, his early adulthood had scarcely been easier. At 20, he was diagnosed with high-functioning Asperger's: the end of the autism spectrum most likely to go unnoticed. He had learnt to manage it and had finally shaken off lingering internalised shame about his sexuality. He'd abandoned a PhD to pursue his passion – tattooing – with demand for his designs growing. And he was in love. That night, he says now, sitting astride a stool, everything felt perfect.

They turned off Hackney Road and left on to Goldsmith's Row: modern flats and a narrow pavement on one side, trees lining Haggerston Park with a wide pavement on the other. Leaves and branches obscured what little street lighting there was. They walked on, chatting intimately.

Stewart and Matt, aged 29, had met in a bar the previous Christmas: instant connection, fleeting kiss, numbers exchanged. Matt had only been with women before. Stewart helped him come out, introducing him to the joys of modern gay life in the capital: freedom unimaginable to many, a minority in exuberance. Three days before their nighttime stroll, the Queen had signed into law the Marriage (Same Sex Couples) Act.

Stewart did not know that this magic was about to be interrupted, that a few minutes of savagery would gradually dismantle his life. How could he? We are never told this story. We hear only of violence, the act. Tales of homophobic hate crimes are reduced to three words: Gay Gets Beaten. Reports feature only the rock, never the ripples. He returns to that night.

In hospital, Stewart felt he had to cover up his tattoos, and his sexuality (Benjamin McMahon)

"I noticed three kids loitering on the other side of the road – from their bodies and stature, I'd say mid-to-late teens. They weren't doing anything but I must have sensed something because I thought, 'Just keep walking'. Then I saw three more kids walking towards us. Suddenly the first three were coming up behind and that's when I knew: this is orchestrated and it's going to be serious." All six teenagers had hoods pulled up over their faces, which, with the darkness, masked their faces entirely.

"As they approached I realised we were not going to escape, so I looked at one of them and said 'Don't' in a pleading voice. But they didn't stop. He shoulder-barged me and before I knew it one of them had punched Matt hard in the mouth, knocking him out the way so that it was just me, surrounded by six people." From his early experiences of being bullied, Stewart knew what to do now: protect vital organs.

"I knew I couldn't fight off six people and couldn't stay standing, giving them full access to my body, with the possibility of being knocked to the ground and my head hitting the curb. So I dropped down on to my elbows and knees." He climbs off the stall to demonstrate the position: head face-down, foetal, hands around crown.

"They started kicking – my ribs, my back – again and again. Winding me. And then they started stamping. All six are on my back, stamping on me." When he rolled on to his left side, they started stamping on his right. "It was all done in total silence. Not a word uttered." They made no attempt to search his pockets or rob him. "They were out for a thrill, and we were two gay men being affectionate to each other: an easy target."

He did not know where Matt was, only discovering later that as they jumped on him, his boyfriend tried desperately to get into the circle, only to be pushed back. The ring on the hand that punched Matt had burst through his lip.

"After a while I realised this situation couldn't carry on. I knew that it was only a matter of time before my chest collapsed, or one of them started stamping on my head and that that would kill me." Adrenaline surged. "I started to think clearly, that I'm not going to have a bunch of kids end my life on a road in the middle of Hackney. I wasn't ready to give up."

"I brought my elbows up and knocked some of their legs out the way and scrambled through the gaps I made, running, shouting to Matt as best I could to let him know I was out the other side. He ran round and grabbed me, we started shouting for help, making as much noise as possible so people in their houses would hear us."

Spooked by the noise, the attackers ran in the opposite direction. Stewart and Matt kept up the commotion, hoping customers at the nearby Little Georgia restaurant would come and help. The diners carried on eating. Stewart dialled 999. The police arrived and stopped at the other end of the street.

"So rather than the police come to us, we had to walk down the street we had just been attacked on," says Stewart. "Matt was bleeding so much he thought he'd lost his teeth." Stewart was having trouble breathing – he assumed he was still winded. "Then one of the officers asked, 'Well, why were you walking down that side of the road anyway?'. It felt like we were being blamed."

Neither the police nor paramedics, in an estate car, offered to take them to A&E – Stewart assumed this was because the casualties from Lovebox would make the wait for an ambulance too long. Instead, they were told to walk to Homerton Hospital. It is nearly two miles away. There, while waiting to be seen, Stewart started feeling a bubbling sensation in the top of his chest, but all he could think about was Matt.

"I was scared for him – he'd only been out of the closet since Christmas and suddenly he's having to deal with this ugly side of being gay. I felt responsible, as if I'd pulled him into this world with its daily risk of street violence."

Matt was sent to another hospital for his lip to be treated. Stewart was kept in overnight as he was still not breathing properly. He detached mentally. "I went into a kind of autopilot," he says. He was also preoccupied with another thought: the assailants would never be caught. As it was so dark, their faces were covered and their clothes had no markings, even giving descriptions to the police was impossible, let alone identifying them.

Ten months after the homophobic attack, Stewart split up with his boyfriend, Matt (Benjamin McMahon)

"From day one of the case I knew I was f****d," he says, looking out of the window. The next morning, after several X-rays, the doctors had news.

"My lung was punctured. It was at half capacity. It had detached from the top and crumpled in half." The hospital admitted him to try and save the lung and keep him alive.

"They put a hole in the side of your chest and go in with a wire, which feels horrible, and feed a thicker tube over that, both of which remain attached to a box beside you, which stops the air from remaining in the chest, helping to re-inflate the lung. They said if the tube became detached or if someone knocked it and it fell out I could die within minutes."

He was put in intensive care. And then in the critical care ward. The man in the bed next to him, whom Stewart believed to be drunk and homeless, picked up the box. Stewart wrestled it off him, and asked to be moved. There were no other beds. For the next two weeks, he remained in hospital. Matt, recovering well from his burst lip, came every day and sat with him. But it was here that the first ripples began: the shame instilled by the bullies seeped back in.

"You construct a bubble around you, with people who make you feel safe, a life removed from homophobia, and then you find yourself on a hospital ward, vulnerable, surrounded by old straight men and their families and nurses who subtly reveal their attitudes towards you as a gay person – one of them stopped talking to me when she realised Matt was my boyfriend. Suddenly I felt I had to be discreet again. I felt threatened. I found myself covering up the [homoerotic] Tom of Finland tattoo on my calf. I found myself slipping into gender-neutral words like 'partner' around medical staff. The whole situation, after the attack, made me think, 'Maybe I should be that person who shuts up and isn't intimate with his boyfriend in case it makes others feel uncomfortable'."

In hospital, the police came and took a statement.

"But they said, 'If you can't identify them there's not a lot we can do'. I don't even know if the statement I gave was coherent because I was on morphine." It would be weeks before Stewart spoke to the police again.

"I phoned every day, I could never get through." He was offered an LGBT (Lesbian, Gay, Bisexual and Transexual) liaison officer. "But I didn't want to be pigeon-holed, I wanted to be treated as the victim of a violent crime who happens to be gay, not palmed off. They didn't explain how that would affect how the case went, or what constitutes a hate crime – what that really means. I felt like they were saying, 'Do you want to play the gay card?'."

Stewart was left unclear how the crime had been categorised. The Metropolitan Police confirm to me it was eventually recorded as a homophobic hate crime, but that no one was arrested or charged. They confirm, too, that there was a period of approximately three weeks in which Stewart was unable to reach the officers involved "partly due to annual leave". They add: "The Metropolitan Police Service is committed to tackling all forms of hate crime". The attackers remain at large.

Out of hospital, Stewart's relationship started unravelling. Matt, physically recovered, and coping better, was able to go back to work and carry on. But Stewart suffered daily with his collapsed lung. An operation to restore it was cancelled.

"The surgeon said to me, 'It's not as though someone's going to kick you again'." For months he remained exhausted, breathless, losing clients from being off work, whole days spent in bed. What should have still been the honeymoon period of their relationship became like a troubled retirement.

"I loved Matt but I felt such a drain on him and so dehumanised by everything. He said he wanted everything back to how things were before the attack and I'd be there with my damaged lung saying, 'I don't know what I can do about that'. The last thing I could think about was being young and free and fun." Anxiety was also setting in. Stewart became depressed, "a recluse", refusing to go out.

"I was terrified to walk around. It was one of the reasons I stopped going to clubs – I was scared of having to walk home." Because Stewart has multiple tattoos – even on his hands – he feared they would make him more recognisable if his attackers saw him again. In public, he ensured his hands were always in his pockets. He didn't wear the jacket he had on that night for months. In November, Stewart and Matt went to Berlin for Matt's birthday.

"On the plane I felt my lung pop, but I didn't tell Matt, I didn't want to scare him and I felt like he resented me already, like, 'Why aren't things fun any more?'." As the plane descended, Stewart lost consciousness. The same thing happened on the way back to London. Finally, in April, he went in for surgery to restore and reattach the lung. It was cancelled at the hospital. He lost more clients, and more money. He went in again for surgery a month later.

"It's an open-chest surgery – they pull your rib cage apart. They pull the lung out, inspect it, and then put it back in and blow it up like a balloon so it sticks." The operation, although successful, left him with three cracked ribs. He needed help even sitting up in bed. It left him with scars on his chest and side and back. He lifts his T-shirt slowly to show me.

"I became so ashamed of my body and so repulsed by my health I didn't want him to see it. Because I'd been opened up my body felt like a carcass, like it was just a piece of evidence, testament to what happened. I didn't just feel like a burden but was physically becoming one. By this point we're already falling out of love with each other..." Stewart stops and, for the first time, breaks down. Tears spill down his face. His shoulders crumple. In May, 10 months after the attack, they broke up.

"Everything became so practical – the next hospital visit or whatever – and so hazardous that it throttled the love out of the relationship. We'd been kicked to death." Matt did not want to take part in this article. Stewart hasn't been with anyone else since. He doesn't want anyone to see his scars, or, worse, ask about them.

"My dreams of marrying someone I loved suddenly got put by the wayside. Instead, I was back to being scared to walk down the street. I felt like all the progress I'd made as a gay man was a mockery of itself. I was having to pretend that everything was OK, and as much as everyone knows you're not, no one wants to hear it after a year. It's old news." Stewart's relationship with his family broke down, too.

"It took my mum a week and a half to visit me in hospital. Two days before my operation, when I phoned, they were too busy to speak. They said they would try and phone later. When they did come, they were late because they went via McDonald's. I thought, 'Get away from my bed. I don't want anything to do with this'."

Stewart still struggles physically. Although back to work at the tattoo parlour, he still can't lift heavy objects, he still gets tired, his lung still doesn't feel normal. "Every time I run or jump off a step I can feel the difference in my chest. I feel repulsed by the scars." And he is left with a strong impression about those who were meant to help him.

"Because it was a physical attack and on a gay man, there was a distinct lack of sympathy from a lot of people, the police, medical professionals. I lost so much – weight, health, mental stability, pride, my partner, all the securities I had built brick by brick I lost because of kids having a few moments of fun. I'm left with a demolished life, starting again. I could have easily died that night. Because things are more open and permissive now, with gay marriage, the general public thinks gays are having the time of their lives. Some days I want to stand in the street handing out leaflets saying what it's really like being a gay person."

In London alone, in the period 2012-2013, there were 1,008 homophobic hate crimes reported to the police, a quarter of those nationally, and a reduction of 20 per cent from the previous year. But is that because people are becoming less inclined to report such attacks? Galop, the anti-hate crime charity, says "most" anti-gay attacks go unreported. I ask on social media for gay people to describe their experiences of police reactions to their attacks. The response is wildly mixed.

"Hammersmith police were really good," says a Londoner.

"I've been attacked twice," replies another. "Both times police showed incredible inability. I told a policewoman about the homophobic verbal abuse. She asked if I was gay. I said that they perceived me as gay. When I called a few days later, the case had been closed. The reason – it wasn't a homophobic attack."

"Brighton police have been disgusting towards my complaint," adds another. "Made me feel like I was wasting their time. They couldn't give a f***."

Ben Cooper, a Labour councillor, emails to say he has made a complaint to Devon and Cornwall police about the way they handled a homophobic attack on him last month that left him unconscious. His attacker, he says, was "let off by writing a letter apologising to me for 'bad judgment'." His MP, Sarah Wollaston, is taking up the case.

Before I go, I tell Stewart that to my eyes, his scars are not repulsive, not at all. I want to tell him that it will get better, that I know it will from experience, but I know he has to discover this for himself. He looks down at his chest where two of the scars lie beneath his T-shirt, and says something quietly that all victims of hate crimes will understand.

"Even when it fades it will be there for ever".