IMO, fairly major surgery on one's psyche is an important part of optimizing one's life. You can't change everything all at once, but a whole lot of attitudes and aliefs can be reprogrammed over time.

For this one, gratitude journal is a good recommendation. Also, the same advice as other habit-changing: learn to notice when your feelings don't match your beliefs, and backtrack to the causes. Repeated self-talk (both affirmations and framing reminders) has a pretty strong effect on me, but others report mixed success.