The Pretty Little Liars star, 28, says that in high school, she used to deny herself food as a form of self-punishment.
"I started self-harming when I was a junior. I would withhold food or withhold going out with my friends, based on how well I did that day in school," Bellisario tells Seventeen.
"I didn't know what was right and what was wrong, so I think I created this bizarre system of checks and balances to create order in my world. But it really backfired."
Bellisario can still be self-critical; she says she often feels like an impostor while on set.
"Sometimes I feel like I'm trying too hard, like I don't belong. I just look around at Lucy [Hale], Shay [Mitchell] and Ashley [Benson], and I'm just like, 'Why am I on this show?' Sometimes I've felt like a fraud."
Bellisario admits that, when she feels low, remembering the love she shares with those close to her lifts her spirits.
"My friends and my family – the people who I love and who love me back. Whenever I get down, when I want to crawl under a rock, I just look around at them and I see how rich my life is," she says.
"You have to remember what's most important in life. I am loved by so many people and have a wonderful job. I know I'm incredibly blessed. I am a completely lucky human being."