Ok...
It may help you to know that
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.
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 isn't landing a 747 or performing neurosurgery - there are plenty of opportunities to recover from my mistakes. Voices in my head, shut up already!"
