The actress admitted that she lost confidence in her looks from the age of 10.
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 Seyfried felt self-conscious about her appearance, she explained that she loved performing songs at home.
The 25-year-old added: "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!"
