Three years ago, Christie Martin was 24 years old and weighed 35 and a half stone. She collected her groceries in the middle of the night from 24-hour stores; she never lingered at the school gates, in case it embarrassed her daughter: she was a complete outsider.

Then she was warned that she could be dead within six months, and with a sense of resignation, she decided to undergo a stomach bypass operation, a procedure which could have been fatal for someone of her bulk. There were post-operative complications and a month of painful convalescence but the weight dropped off at the rate of more than a stone a month for the first year and today, Christie, from Arkansas, USA, is, at 12 stone, a third of her former size and is still, slowly, shedding the pounds. She has yet to relinquish the self-loathing of the seriously obese, but life is finally moving on and, on the surface at least, Christie's story is a personal triumph.

So how did Christie get so fat in the first place? She says that she came from a destitute and broken family and was abused, mentally and physically. By the age of eight, she was nearly 12 stone. "Every night, I prayed I wouldn't wake up in the morning, but I had to act like it didn't bother me.

"At 15, I went to live with my father and he didn't buy groceries but made us live on Slimfast. Then I'd go round to my mother's house and eat supper there and she'd say: 'Slow down.' Everyone made me feel uncomfortable about eating so I'd try to eat the way they wanted me to, but when they weren't looking I'd eat anything that was available. I did most of my eating secretly. When I was made to clear the table and put the leftovers away, I would take them into the kitchen and stand and eat it all real quick. I felt hungry and ashamed."

As she grew up, Christie's relationships with men were often abusive: her weight made her feel she deserved nothing better. "I think obese people settle for the leftovers." At 19, she gave birth to her daughter Olivia, by her first husband. They split up when Olivia was two. A year later she met Robert, who was to be her second husband. (She admits she stayed with him so that the £16,000 cost of her surgery would be met under his medical insurance.)

Christie's weight severely limited her life with her daughter. "Other people had to take her out. I tried taking her places. I couldn't get into the car at the fairground but I could push her on the swing until I got out of breath and every part of my body started hurting." There was also the problem of shielding Olivia from other people. "People would stare at me or chase me in their car and shout: 'Why don't you die, you big fat fuck?' As I took a step, teenagers would should: 'Ba-boom, ba-boom.' Olivia said once: 'Mummy, that's mean. Why are they doing that?' I began going out less and less."

Finally Christie was referred to Professor George Khan at the Baptist Memorial Hospital, Memphis, Tennessee, who believes that extreme obesity can be cured only by surgery. Christie was suffering from sleep apnoea, in which the tongue slips down the throat, threatening suffocation and causing the sleeper to wake continually, gasping for breath. To allow her to rest, she was sent home with her windpipe cut open for three weeks before the stomach bypass.

Tonight an ITV documentary will chart her struggles during and after her operation. In the film, Christie is seen saying goodbye to her daughter before the operation. "I'd have done anything for Olivia and without the operation our days together were definitely numbered. It was scary but I knew she would be taken care of if anything happened to me," she says.

In surgery, Christie's stomach and intestine were divided so that all food must pass through a half inch pouch. After a minute portion of food she feels full. "I still pile it all on my plate but have to throw it away."

After she had lost 19 stone, she had 15lbs of loose skin cut away, giving her waist definition for the first time. "I felt attractive to men but I had such low self esteem I had to get a buzz going first, by getting a few beers down me." She also turned to drugs, including marijuana and cocaine. "I had a new body but I didn't know who I was anymore. The weight came off so quickly I couldn't deal with it. Losing weight made me feel better for a while. Wo! It was wonderful. But then I realised I was still sad and miserable and wanted to die. I had been hiding behind the fat. It had been an excuse for not going out and not dealing with my problems."

Christie is currently living in a shelter for the homeless since leaving another abusive boyfriend. She has been offered a three-bed, two-bath flat on a government assistance plan and hopes to find work soon. Tragically, Christie's problems are being replicated in Olivia, now eight, who, in the Carlton film, is seen gaining weight. "It's getting hard to buy clothes for her. I tell my family not to goad her. If they make an issue of it, it'll make her want to go and eat. I'm trying to keep her active. We go walking and play in the park.

"She was passed around my family so much and now I'm trying to rebuild our relationship. I was mentally abused and sometimes I say mean things to her. I get mad and say: 'God! You're so slow' or 'Go ahead, get fat. Be like I was', then I ask her to please forgive me. It is a long road for both of us."

• Christie's Story is on ITV tonight at 10pm.