Here's an interesting idea. Get a box. Put the good things people have said about you (or your work) in it. Maybe print them out on small pieces of paper or write them longhand on note cards. You could write down pleasant past memories too--the little things that you have trouble remembering. Put them in the box.
When you get depressed, pull one of these out. Read it. And (hopefully) you'll feel better about whatever you were sad about.
I'll try it (once I get a moment) and tell you if it works.