Debbie Downer

(noun) a pessimist, one who looks on the downside of things, has zero confidence in themselves, criticizes themselves, sees the glass as half empty, you get the picture.
kind of like Grace Dimond

Grace Dimond: Ugh, my art project stinks. I'm going to fail. I suck at art. I will never get into college. I am so bad at school. I fail at life.
Julia: Grace Dimond, you are such a Debbie Downer.