The opening photo in Half, Julia Kozerski's series of naked self-portraits, is actually the bookend to a sequence of earlier photos. In those, she appeared unhappily in her wedding dress in a changing room cubicle, more than 300lb (21 stone) and mortified. Here, she appears in the dress again, standing sideways on to the camera, to show how much of the dress is unoccupied. Over the course of a year, Kozerski lost half her body weight, and you might expect the resulting photos to conform to the glib narrative of before and after. Instead, the 28-year-old took pains to show "what real is, what raw is" – in this case stretchmarks, skin folds, contours like sand dunes. Raw is Kozerski naked, and frequently crying.

Nudity is an overused gesture in photography, particularly when it purports to "celebrate" the "ordinary". You can't turn on the TV (Lena Dunham), go to a gallery (Spencer Tunick) or, if you're in San Francisco, enter a civic building these days without tripping over someone getting their kit off in the name of corporeal democracy. That Kozerski still manages to be shocking and interesting is testament to her ideas and her courage. The question most people ask on seeing the photos – after "Why don't you get surgery to remove the extra skin?" – is "How did you get the weight off?" which she thinks misses the point. Losing the weight was tough, she says: "I had no idea who I was, and while I went through all that I was lost." But what came after was tougher. Contrary to media everywhere, being thin isn't enough of an identity to go on. "This is it!" she thought, when she finally got her weight down, and then: "Now what do I do?"

We are in a coffee shop in Milwaukee, where Kozerski grew up and where, after finishing her degree in fine arts, she works in marketing. The photos, taken when she was at her most vulnerable, don't prepare one for how she is now. Kozerski could advertise the midwest: she is fresh-faced, ruddy-cheeked, brimming with enthusiasm. "Nothing fancy, I'm from Milwaukee!" she says cheerfully. She is a regular weight, she points out. Not model-thin, but the size that, after a lot of trial and error, she worked out she needed to be. When she decided to lose weight, she signed on instantly to the cult of perfectionism. She thought, "I'm going to be this amazing person, I'm going to be a model! And that's not what happened. It was a transition into something new; into learning to love myself as I turned out, as I was and as I am now."

"Loving who she was" sounds like a dictum from therapy, which Kozerski didn't have. She didn't hire a personal trainer, or join a gym. The most remarkable thing about her year-long journey is that it wasn't assisted by exorbitant lifestyle aids. When I ask to whom did she outsource her motivation, she laughs and says, "No, no, no, no one." At least, no one beyond her husband, Tim, a mechanic who cheerled his wife from day one and was hugely understanding, even though "he can eat whatever he wants and nothing ever happens to his tiny little body".

Instead of paying someone to shout at her in her lunch hour, Kozerski walked the dogs. She exercised "portion control". She cut back on fizzy drinks. "I feel like it's a disappointment to people, they want some magic remedy and I don't have one." When things got really tough, she looked at those photos of herself trying on her wedding dress at the one shop in Milwaukee that carried her size. "It's humongous," she says. "And I still put it on. But these days my husband can fit into it with me."

Growing up, no one in Kozerski's household was thin. She is one of three girls and their parents worked hard, her father in sales, her mother as a librarian and teacher in the public school system. "There was a lot of frozen food after work, real quick. We drank a lot of soda. We didn't have specific meal times, so I'd be eating throughout the day. It catches up with you."

There is almost no period in her memory when Kozerski can't recall being sensitive about size. When she was in junior school, she avoided the girls and hung out with the boys. "The boys would accept me for who I was. I'd play sports with them, they didn't care what I looked like. But the girls were always into fashion, and I knew I couldn't do that."

Things got worse as she got older. Shopping was a nightmare, particularly for big events. For her prom, as for her wedding, she shopped alone, as quickly as possible, and bought the first thing that fitted. "I got the bare essentials and got out."

Kozerski's mother died in 2011 of diabetes. Her father has had a triple bypass. There are long-standing weight-related health issues in her family. Before her mother's death, Kozerski asked her why, when she saw her three daughters getting unhealthily large, she didn't say anything. "And she said, 'I never wanted you to be unhappy; I never wanted you to think that you had to be thin.' And I said, 'You know, I was unhealthy, I could have died, you should have said something.' And she said, 'I would rather us have a good relationship than have it strained by me telling you to diet.'"

Kozerski sighs. "It's not their fault; they didn't do it out of ill will. They were unhealthy, we were unhealthy, it was just our lifestyle."

It wasn't the wedding that triggered Kozerski's desire to change, although she anticipated the day with a certain amount of grim humour. She and Tim met when they were 15, in high school, and although the actual day of the wedding was wonderful, "I knew I was going to be the big girl bride with the skinny groom. And I knew what the pictures were going to look like. Reliving that is tough."

It was six months later, as she and Tim sat idly on the sofa eating cookies, that Kozerski suddenly had the urge to weigh herself. She got up and went to the bathroom. "And I saw 338lb. And I thought, 'Oh my gosh; if you think that a model is 100lb, all of a sudden I'm three people!' So I started freaking out. I thought I could die. I thought, 'What does it mean if I have children?' So I said, 'OK, I'm going to carry on eating cookies tonight and then tomorrow, I'll start.'"

A thousand resolutions like this are made every day, but Kozerski meant it. So alien was the concept of nutrition that the next day she sat at her computer and typed, "How to eat healthily" into Google. She devised methods of incentivisation, cleverly realising that small goals would be more effective than huge, unattainable ones. Instead of focusing on an end weight, she worked in increments of 10lb and no more. With each 10lb she knocked off, she gave herself tiny rewards; a CD she wanted, a movie.

The only weight-loss gadget she bought was an armband that measures how many calories you're burning against what you're eating. She rolls up her sleeve and shows me; Kozerski has burned 1,300 calories today. "I don't think you need this," she says, guiltily. "You can work it out for free on the internet. But I love it."

Having, for most of her life, treated her body as an enemy and tried to ignore it, she started to be sensitive to small fluctuations. "Like when it gets cold out, I don't lose weight as quickly. There are hormonal fluctuations. I just became very attuned to how I functioned. There were weeks when I gained, then lost, then stayed the same. I was beginning to understand my body."

She was also beginning, slowly, to understand that losing weight wouldn't automatically fix all her problems. Kozerski had always taken photos; her degree at the Milwaukee Institute of Art & Design had a photography component. As she documented her weight loss, starting with that horrifying moment on the scales, she realised that as her size went down, she still looked unhappy.

There was a lot of shame in her system. Part of the reason she wouldn't go to exercise classes or ask for help was that she didn't want to involve other people: "Because I felt like it was such a burden. And I also felt I did this to myself." She didn't want to be complimented either because, at that stage, she didn't feel she deserved it. "So people are saying, 'You look great!' And you don't feel great. You are in transition. You think, 'No, no, not yet.' It was never good enough."

At one stage, she went into overdrive and began over-exercising.

"There were times when I walked 20 miles a day and I would come back and say, 'OK, I just spent six hours walking. That's too much.' When you start counting calories, you want to push yourself, just 20,000 more steps, and it becomes addictive. And I stopped when I realised that. There was a picture I took in the changing room in a store and I look very unhappy. I was skinny, and I didn't feel well. My blood pressure was high and my face super-red." (She doesn't like the word skinny, particularly when her husband uses it. "Sometimes he'll say, 'You look skinny' and it really freaks me out. I don't mind the word fat – I wouldn't use it towards other people, but I do say it to myself. But the word skinny bothers me." It raises the spectre of comparison. "Like, are you comparing me to the old me, or to a model? It doesn't mean anything.")

The photo of Kozerski looking thin and unhappy is one she keeps on her phone as a useful reminder not to get any smaller. The naked photos act as its counterweight; a decisive gesture towards embracing "imperfection" and a desire not to eradicate all signs of her struggle. She's not so sentimental as to "love" the folds of skin. "I hate the skin." But it's a good visual reminder of how she once was and the choice she made not to be that way any more. When she looks at the skin, she thinks, "I don't want to fill it back up." With some effort, she says, "This is where I came from, it's the baggage to show I'm not trying to be a model. This is a real person. And I like having conversations about it. People say, 'Oh, I have stretchmarks after having a child.' It's not just about weight, it's about everyone."

She calls the photos "grotesquely beautiful", and they are. They are alienating in the best way, acting against the industrialised uniformity of most high-profile women. But, given that Kozerski is on the shy side, how on Earth did she get to the point of putting the images on the internet?

She laughs. As with the weight loss, she took baby steps. First, she took them into college and showed them to classmates. "I felt safe, with a little community of artists, that they would see it isn't pornography or exploitation. So I had that buffer."

Then she submitted the work to a gallery show in Colorado. The theme was food, and she sent a photo of herself naked and demoralised in front of the fridge. "And it won. And it got publicised all over. And then I thought, 'OK, this is it.' I didn't have a choice. It was out there."

For the first time, in Colorado, she started to talk to strangers about the psychology of size and her search for a happier way to be. "I just had the most wonderful conversations with people about the work and my experiences."

Since then, Kozerski has had a lot of exposure. There have been negative reactions, mostly from abusive male posters on websites. What do they write? "Superficial stuff, like 'She's still ugly.'" Kozerski shrugs. "People are brutal. It's out there."

Where does she think the anger comes from? "People are insecure about nudity. And it's not the nude people want to hang over their fireplace."

Mostly, however, she has encountered a huge wave of positive interest from people. (Her dad, she says, would prefer it if she was still taking photos of flowers like she did in high school. "But I think he's very proud of what I've done, especially with the health concerns. I'm improving my life.")

In one photo in the collection, Lovers' Embrace, she lies alongside Tim, her defender and champion. "I thought it might be too personal, and relate too much to me. But he's a symbol for all the people who have been supportive throughout."

She has, at the end of it all, come up with something like a theory of happiness. It rests on two things: to allow for the possibility of failure and "to disconnect from comparing yourself to other people. To figure out who you are."

How do you do that when the entire world turns on comparisons?

"Well," Kozerski says, "when I look at a picture of a model, I go, 'OK, she's pretty. But she's not me, she's not working my job, she's not married to my husband, she's not living in my house.' So," she smiles at her luck, "it doesn't matter."