You've got to work on challenging dysfunctional thinking. I recommend CBT for that.

I know when we're depressed that if we had to list a Pros and Cons list of things in life, the Pros list may be shorter than the Cons. But that is the time when you make yourself remember when you did feel well. Hey! Try to ask yourself this... When you did feel well, what would you have put as your Pros vs. Cons? I bet the Pros in life list would be longer. Maybe next time you feel better, make that list and keep it in your dresser drawer and pull it out when the glass seems half empty instead of half full.