Are writers and authors too self critical? Are they often too self doubting? In a simple answer, yes! Of course every individual is different, but most of my friends who write are their own worst critic.
This doesn't have to be a deal breaker though. I know that I am hard on myself and my writing, but I also know that when I push myself to rewrite and make my work acceptable to that harsh inner critic I produce better work than I otherwise would have.
On the other side of the coin the writer's self doubt I have is less productive, and it's honestly not at all helpful. I cannot help it, I always wonder if I suck. I entertain the notion that I am wasting my time writing when I could be doing other things that I may not suck at so fantastically. The self doubt can get to me and block me, or it has even prevented me from writing for days, even weeks.
This inner bully needs to pipe down most of the time! Critics can be helpful and point out areas that need change. Doubt has no focus in change, no means of helping or improving. Critique has it's time and place. Doubt can go suck and egg.
My advice to writers all around (epically myself)... Try to work with the self critiques, and duct tape the big mouth of the inner doubter :)