So many of the posts about fear have been interesting and thoughtful. In thinking about the issue, most of my fears about writing come down to the question of is it good enough. The initial fear of is it good enough to attract the interest of an editor is replaced by is it good enough to pass through an acquisitions process, and then if that happens, is it good enough to be a book. Unlike other aspects of my life where I have repeated chances to improve something, once a book is published, that is the book.
I find myself being aware of these fears and going ahead and doing the work. The moving forward isn't an absence of the fear so much as an acknowledgment of the fear and a decision to just do it. I've heard writers of over one hundred books describe the fear that they won't have anything to say or the work won't be any good so I don't expect the fear to disappear.
As others have said, the fear of a story not being good enough provides a motivation to make it better, and each piece can continue to be improved over and over. At some point, we let it go and when it becomes a book, we live with it in that form even though we know it could be better.
So over and over, I greet the fear, sit down and go to work.