Certainly the most destructive vice that a person can have, more than pride, which is supposedly the number one of the cardinal sins, is self pity. Self pity is the worst possible emotion anyone can have. And the most destructive. It is, to slightly paraphrase what Wilde said about hatred, and I think actually hatred's a subset of self pity and not the other way around -- "It destroys everything around it, except itself."
Self pity will destroy relationships, it'll destroy anything that's good, it will fulfill all the prophecies it makes and leave only itself. And it's so simple to imagine that one is hard done by, and that things are unfair, and that one is underappreciated, and that if only one had had a chance at this, only one had had a chance at that, things would have gone better, you would be happier if only this, that one is unlucky. All those things. And some of them may well even be true. But, to pity oneself as a result of them is to do oneself an enormous disservice.