As a writer, I find that there are times why I struggle with self-doubt. I'm sure that this is true of most writers, and there is no real cure on the market for it. It is simply something that happens from time to time.
For me, the time usually occurs when I am struggling to finish a book and I feel "stuck." It also happens when I read a really good book and question my talent in comparison to the talent of the author I'm reading.
It also happens when I really tired.
But I know that these moments of doubt will be short-lived, and my mind will straighten itself out again eventually. It's all a part of the profession that I have chosen. No, correction, the profession that chose me.
So I will muddle on, and try my hardest to be patient. I will continue writing, and when the current project is complete, I will look back on these troubled times and realize I was just being over critical.