The `Jennifer`s Body` actress struggled to accept her looks when she was a teenager and became nervous around other people.
She said: "I was super-outgoing until I was around 10. I got a bit older and started getting shy. Way too shy.
"I felt so extremely ugly. When I look back, I was not ugly - I was cute and had a gap in my teeth. But I wish I could have enjoyed that part of my life and be more confident."
Although Amanda was reserved when she was in a group, the blonde beauty used to love performing tracks from her favorite musicals when she was alone at home.
The star sang so loudly she terrified her pet cat, who avoided her when she was belting out show tunes.
The 25-year-old actress explained: "Sometimes I would sing around the house for two hours straight, but I didn`t want anyone to hear me up close. I would sing all the Broadway show tunes like `Cabaret`.
"Our cat started hating me. He went from being affectionate to being almost scared of me. Our theory was that when I was singing around the house, I hit notes that hurt his little ears. That is what I tell myself anyway!"