Ok...

It may help you to know that

1. Women are much more prone to this sort of feeling than men *for no competence related reason at all.* It's just that society had conditioned you more to doubt yourself, to try to be perfect, and generally to introvert more - which is often a good thing, but not in this particular case.

2. Competent people are much more likely to worry about their competence than incompetent ones: http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

Both these things are irrational! Accept that you feel this way and then say: *"So what? Frak it, I'm going to get up there and teach. I may not be perfect, but I'm going to make a solid plan and follow it, so rationally, nothing can go too far wrong. The students aren't idiots: if I miss something important, one of them will ask, and that will cue me to tell the others. This is landing a 747 or committing neurosurgery - there are plenty of opportunities to recover from my mistakes. Voices in my head, shut up already!"*