Confidence Problems .. Help Please ?

So I'm 16 , and I've been having confidence problems a lot . I don't feel pretty , and I just don't think I am. Every time I walk past a mirror, I try not to look in it because I don't want to see myself. Whenever someone says I'm pretty , I feel like they are just being nice. I just don't ever think I'm good looking. I always think theres something wrong with me . Idk what to do , please help ?