You may have a case of the "Imposter Syndrome,"
U.S. News columnist Rebecca Thorman summarizes the situation
Imposter syndrome happens when you don't feel good enough. You're afraid that at any moment you will be found out. You feel unsure of what you're doing, that you don't have any expertise, and that you've just been "faking it" all along.
Read Rebecca's six strategies for overcoming the Imposter Syndrome here. We particularly recommend #2 and #3 -- especially useful when you have any set-backs!