A bunch of kids from my theory class were trying to weigh the sincerity of compliments. And the validity of standardized testing. We all ought to be confident enough that we're supersmartypants and besides, it doesn't matter. But how do you know if you're smart enough? Compare and contrast.
Your professor thinks you're brilliant. Your professor thinks everyone's brilliant.
You got a good GRE score. Someone in your class complained about a 780.
Your friends love your writing. None of your friends are major journals' editors.
You do some things well. You don't do everything perfectly.