I enjoyed doing the gag covers better than the story ones because they were usually simpler. A cover based on an incident in the plot took a great deal of staging to tell a little story that was still part of the book. And it had to make sense on its own.
One day I think it's the greatest idea ever that I'm working on. The next day I think it's the worst that I've ever worked on - and I swing between that a lot. Some days I'm very happy with what I'm doing, and the next day I am desperate - it's not working out!
I find that once you start helping others, it makes you feel better about yourself. It helps you figure out what you want to do with your own life.