The 18-year-old star checked into rehab – where she received treatment for depression and an eating disorder – in October following the altercation with Alex Welch and admits she had been "out of line" for several months beforehand.
She said: “I was completely out of line all summer. Just the worst attitude—totally ungrateful… I just felt like she’d betrayed me. That’s the bottom line. When you punch someone on a plane, enough is enough. Right after, I texted my mom and just said, ‘I’m sorry.’ ”
Demi insisted her problems didn't stem from her split from Jonas Brothers star Joe Jonas, saying she had a lot of "other stuff" happening.
She told America's Elle magazine: "I wouldn't credit my meltdown to a guy. There was so much other stuff in my life."
The 'Skyscraper' singer also opened up about the bullying she received at school, admitting she was branded "fat" but couldn't understand why she was being targetted.
She said: “Those girls never gave me an explanation for why they were bullying me. One of the words they called me was fat. At that age, you think, 'Oh, so I don’t have friends because I’m fat.' ”