Why do I always want to change who I am and how I look?

I know it sounds weird but.. I'm always looking to change who I am and how I look. People are always telling me how pretty I am, but i just don't see it. I am knocked kneed, i have hereditary bumps on my legs and thighs, I'm always thinking about ''when I'm older I will get plastic surgery for... '' Our society is so appearance based it's hard for girls my age to be comfortable with ourselves.