When I was a teenager my mom told me the same thing... so I joined a pageant and won just to prove her wrong.... then told her see you were wrong... of course, the scars of what she said stayed with me for years, and deep inside I felt like it was a fluke I won, I kept thinking that it was a mistake, they felt sorry for me etc....

Hang in there, if you felt good about how you looked before your Mom said what she did, then try not to let her poor self esteem take your self esteem down... I know that is hard....
I am so sorry you are going through this pain