[j. d. worthington]

I'd agree that it's quite common. Moorcock has mentioned that feeling of depression after finishing a book, before his spirits pick up again and he can be actively engaged with the next; Lovecraft almost always felt what he'd done simply didn't live up to what he wanted, and in fact disavowed some of his best work (such as The Case of Charles Dexter Ward). The list of writers who have felt this way is, if not quiet endless, quite lengthy; so I wouldn't fret about feeling this way; it's simply part of the process....