It's interesting to read that other people suffer from the same problems. I'm also a 'self-doubter' as you put it. I find myself with four or five chapters and when I get stuck, instead of writing down whatever comes into mind, I go back and edit. So now, I'm without any new chapters but I'm also starting to think the ones I have aren't good enough.
In my personal experience, I accomplish more if I don't go back.

Good luck with the writing!