can this friendship be saved?

For five years, I've been good friends with a girl I'll call Debbie Downer. We have a lot of things in common. We both work in magazines, we like the same kinds of movies and TV shows and books. And on a deeper level, we just "get" each other. But ever since I've known her, she's been super depressed. I can't count the number of times I've told her, "Your life does NOT suck. You are NOT fat and ugly. You are NOT going to die broke and alone!" No amount of affirmation or advice seems to help. I just hate to see her so unhappy and I feel really helpless and depressed when I'm around her. But I don't want to "abandon" her, you know? Got any advice for dealing with a toxic friend?