I think all authors are afraid of that in some way or another, no matter what they write. Are my characters solid, are they three-dimensional? Did I take my time and flesh them out enough?
Some scenes I've written make me cringe and I've been known to ask my editors over and over, "do they read okay?" lol I cringed with each and every chapter of the fic I posted here, even though I had been pretty proud of myself previously.
Shadowknight (edit: ack, sorry not used to this forum setup!) basically has the best advice, IMHO. Detach yourself from your work and you'll be better for it. Even negative reviews and flames can help you if you keep an open mind and your emotions in check.