Tyra Banks: 'I know the pain of being too thin and too big'

Tyra Banks is not one to keep her self-doubt a secret.

When the supermodel and her mother, Carolyn London, launched a program in 1999 to mentor young girls – the TZONE Foundation – Banks realized the importance of sharing the insecurities she experienced while growing up.

"The summer I turned 11 years old, I lost 30 lbs. and I had a growth spurt," Banks, 40, tells PEOPLE in this week's issue. "And it wasn't even supermodel thin. It was more than that. Some people thought I had an eating disorder." She didn't.