"It took a while until I felt comfortable talking about it," she tells PEOPLE. "That's one of the reasons I decided to do this campaign: to raise awareness that binge eating is a real medical condition."
Seles, who is a paid spokesperson for Shire, says she developed the disorder as an adult while dealing with pressures of being an athlete and her dad's battle with prostate cancer, as well as trying to recover from being stabbed on the court in 1993.
"My eating was just uncontrollable," she explains.
"My binges would be potato chips, pretzels and cookies – bags of them," Seles, 41, adds. "It's really eating huge quantities of food in a very short period of time."
Seles, whose weight fluctuated throughout her career, says it was difficult for her to comprehend the condition, which is the most common eating disorder in U.S. adults, affecting more people than bulimia or anorexia nervosa.
"It was very hard to understand how on the tennis court, I would be so focused and so disciplined in my training, but when it came to binge eating I had zero control," she says. "I felt really embarrassed about it."
Seles decided to seek help and says she felt "such a big relief" when she was finally diagnosed.
"Now I feel comfortable eating in a social situation and I don't feel the urge to go back to my house or hotel and binge eat," she says. "But I still have it so it's really about learning to manage it."
Correction: Feb. 18, 2015 – An earlier version of this story omitted the relationship between Monica Seles and Shire.