Regarding impostor syndrome, I was interested to read that John Green and Neil Gaiman felt the same sorts of things. I absolutely have times I think my books have been nothing but a disappointment and I don't deserve to be where I am whatsoever. Generally I try to welcome that feeling because it pushes me to try and do something better and prove myself, but it's also disheartening because I know the cycle will never end no matter what I accomplish. I think everybody feels that way to some extent.
Yes, I think it's good to know we're not alone feeling that way, and even really famous writers/actors etc feel the same way sometimes. And I agree with Shoeless and Jean --- I feel best when I'm working on the writing itself. I don't worry about what people will think about it. But there's that moment of vulnerability when the writing has to be shown to other people, and that takes some getting used to. It's a little like pushing a baby bird out of the nest, hoping it'll fly.