12/28/2011 at 11:00 AM EST
These days, slipping into a pair of size 2 skinny jeans isn't a challenge for Miss South Carolina
, Bree Boyce. But for most of her life, Boyce says she was "miserable in my own skin" as a heavyset kid growing up in Florence, S.C.
"I used to make fun of my weight with others to fit in and be the funny, overweight girl," Boyce, 22, tells PEOPLE in this week's annual Half Their Size cover story.
"I'd go home and cry, not because of what others had said about me, but about what I had said about myself," she adds.
By 17, she had reached her highest weight of 234 lbs. Aching knees and difficulty breathing drove her to meet with a doctor, who told her she needed to lose weight immediately.
"It was embarrassing because a 17-year-old shouldn't be sitting in a doctor's office having this kind of conversation," says Boyce. "I knew there were so many things I wanted to do in life – I wanted to be on Broadway, go to school, get married, have a family."
Another key motivator? "I wanted to be Miss America one day."
So the teenager started keeping tabs on what she ate every day and constantly reminded herself that food was fuel and "not comfort." She gave up a sedentary life and began taking exercises classes two or three times a week.
After three years of sticking to her new lifestyle, Boyce dropped 100 lbs. and competed for the Miss South Carolina title in 2010. The following year, she had lost 12 more lbs. and won the crown on a platform committed to eating healthy and fighting obesity.
"I don't want young girls to worry about the number on the scale," she says. "I'm really promoting health and loving yourself."
Boyce will compete in the 2012 Miss America pageant
on Jan. 14.
For more on Boyce and other half their size weight-loss stories, pick up the new issue of PEOPLE magazine, on newsstands Friday