I think what you are feeling is pretty normal! I had to take a weekend and relearn a bunch of stuff, just to calm myself down. Could you find some web tutorials in the topics you don't feel you have a good grasp on? You might be surprised at what you actually know.

I still get twinges of 'I have no idea what I am doing,' but I try to just ignore them and move on to a new thought.

Have you ever read about imposter syndrome?