Maribel Contreras, 38, 5'6"
El Monte, Calif.
Before: 217 lb., size 16
After: 140 lb., size 6
Total pounds lost: 77
Total sizes lost: 5
My whole life, I was a yo-yo dieter. I would restrict calories to drop weight, only to binge-eat and put it back on. In 2009, after giving birth to twins, I left a job I loved to move to San Gabriel Valley, Calif., and soon turned soda and Doritos to cope with all the stress. In July, 2013, my poor eating habits and lack of movement finally caught up with me; I hit an all-time high of 217 pounds, more than what I weighed while pregnant. Then my doctor told me I was prediabetic; I knew I had to make a change. 
A lifesaving move
Later that month, I joined an online weight-loss support group. After some persuading from friends on the site, I scheduled gastric sleeve surgery. While the operation promised to control my eating and make pounds melt off, I was unable to keep the horror stories I had read about the surgery from flashing in my head. I canceled the procedure the night before it was supposed to happen. My weight wasn't something surgery could fix, anyway—I needed to adjust my attitude. The next day, I joined Gold's Gym and started working with a trainer three times a week.
Routine, revamped!
I exercised regularly and lost a little more than 20 pounds in a year and a half. But I was still usually choosing cheesy quesadillas over more nutritious stuff. So I decided to register for a 12-week body transformation contest at my gym. In addition to revving up my workouts, I swapped out my typical fast-food meals for healthier options, such as grilled chicken and eggs. By the end of the three months, I had not only shed 47 pounds but also won the entire challenge! Knowing that I had transformed my body was the greatest prize of all.
Secrets of a 77-pound loser:
Maribel learned a few secrets along the way. Now she uses them to stay on track. 
Pick a power playlist
Starting off my workouts with pump-me-up songs like "Shake It Off" by Taylor Swift keeps me motivated throughout the session.
Find a mantra
On days when I want to give up, I repeat these words: "I'm very strong; I can do this." Just saying the sentence out loud stops me from feeling weak.
Tech it out
My Microsoft Band ($250; amazon.com) helps me maintain my fitness goals. It lets me log my exercise and calorie burn to make sure I'm hitting my targets.
Broadcast your goals
When I joined the challenge, I told everyone about my plan to win. The fear of losing and having to admit to all my friends that I failed kept me going.
As told to Lindsey Murray