When she first lost weight, Demi's dancing did improve - but soon she was skipping classes because of the condition.
She said: “After about six months, I started to get lots of concerned messages saying I was too thin.
“I told people I had glandular fever, because I was totally in denial about this being a problem.
“My mum, Colette, and my teacher tried to bring it up and tried to get me help.
“But I would flip out and get really angry. I didn’t want anyone to speak to me about it.
“It was like there was another person in my head, who was always telling me what to do.
“It’s like a lion versus a mouse – you can’t stop the lion taking over.”