I've always had a similar problem too, where the mirror is my friend but photos aren't. I think, though, that having confidence in yourself, at least some, is a factor and so thinking you look great isn't always a bad thing. Also, photographs can often be taken in an unflattering manner too--don't forget that!
But I do understand how hard it is to think "is that how people see me?" But I still believe that people read how you feel and take cues off of that. No matter what, good luck in all that you do.