Detective author Walter Mosley ("Devil in a Blue Dress") was interviewed a while back by Tavis Smiley. Here's what Mosley said about why you should be writing--even if you fear you're not good enough:
...Archie Moore was once asked what he thought about Rocky Marciano. They said, "Well, Archie, did you always respect Marciano?" And he said, "I respect every boxer." He said, "'Cause you can never tell at which moment they may burst into bloom." Meaning to say, "When they might knock me out." And it could be anybody. So the thing is is that I don't - and even in that book, in the very beginning, I say "I don't know if you're gonna get this book published. I do know you're gonna write it. Are you a great novelist? We'll find out."
[Note: Mosley's book on writing is "This Year, You Write Your Novel"]
For more on how to write despite the seven fears you may encounter, see my book, Your Writing Coach, published by Nicholas Brealey Publishing in the US and UK, 2007.