I recently hit a rough patch where I thought my writing was awful. I sulked last night about it and wisely stayed off the computer. But this morning, I got right back on the horse and worked on some editing. And y'know what? My writing isn't that bad.
All writers think, at one time or another, that their writing isn't worth reading. It's a natural part of wanting to be a better writer than you already are. Please, if nothing else sticks in your head about the craft--remember this. Write it down, and tape it to your computer:
Tough it out. Don't quit, because the very next manuscript could have been a New York Times bestseller.
Eventually you're going to write something that makes you remember why you sit in front of that damn computer every day and struggle to say something worth saying.
I've said it before, and I will say it for as long as it takes to encourage every new writer to be a better one. No one ever published a quitter. :)