Do you ever have this experience? You're driving in your car, walking, doing something outside your workspace, and you start thinking about your WIP. "There isn't enough indication of character," your brain says. "You have to go back and add more about motiviation." It's so obvious at that moment. Yup, that's exactly what the project lacks.
Then you get back to the computer and start reading, and you find that your brain was wrong. There's plenty of motivation, and aside from a word or two that clarifies the relationship between the subject and an object, you like it just fine.
Who's right? Your brain away from the screen or your brain in front of it?
I think we (well, I) have insecurities that surface at odd moments. (Classic one: "Did I unplug the iron before I left home this morning?") I don't think you'll ever get rid of such thoughts. When you're on task, you do your best work, you focus on putting it all together, and you build the best story you can. When you walk away, your mind starts doubting. It isn't as good as you thought. It's going to need a lot more work. Maybe even: it's terrible.
Rereading helps, especially rereading the whole piece to feel the flow and find the dead spots. Advice helps if you can get readers who care and who have some sense of what you're after.
But in the end, you have to tell those little mind-demons who whisper negativity in non-work hours to shut up. More times than not, they're just trying to scare you, and this business is scary enough without your own head adding to the problem.