"When I was younger, when I was only 14, I was told by a drama teacher that I might do okay if I was happy to settle for the fat girl parts - look at me now," she said.
"And so what I feel like saying in those moments, to any young woman who has ever been put down by a teacher or a friend or even a parent, is just don't listen to any of it."
She went on to say that it took her a long time to shake off her insecurities, but she never let other people's cruel comments stop her from pursuing her dreams.
She told girls watching: "Keep believing in yourself. That's what I felt like I really had to dig deep and do.
"I would dedicate [this award] to all those young women who doubt themselves. Because you shouldn't be doubting. You should be just going for it."