Not getting across the idea you wanted, or not making the statement you wanted, or not making any statement, or not having any idea of what you want to say, or on a simpler note, not producing something that makes you happy, or that you yourself like. It's really easy to feel like a failure in those cases. And when one gets into one of those periods, it can be hard to get over it - "oh, I won't bother, I'm producing nothing but crap anyway." I know I've had times like that, and I'm sure a lot of other people on this site have had 'em too.
Originally Posted by blansky
There's an interesting book I've recently read called "Art and Fear: Observations on the Perils (and Rewards) of Artmaking" (ISBN 0961454733) that talks about this. At least one of the authors, if not both, is a photographer.