I think that I’ve started to think that it’s neither. It’s not innate, and it’s not taught.
I think it’s something that we have to develop. And that’s not to say that others can’t help nurture its development in us—they can and do (and can do the opposite, too). And it’s not to say that it might not come more easily for some than others due to their circumstances or disposition or what have you.
But I think confidence is probably more than anything a result of us experiencing, over and over again, that we will fail sometimes, but we won’t fail all the time, we will get better over time if we keep at it, and none of this has any bearing on our self worth. These aren’t things that can be told or shown or “taught” to someone, I don’t think. That hasn’t been my experience, anyway. If they’re just words, they don’t really mean anything, they aren’t a part of us. I think we need to actually experience this until we believe it. (And it’s in these experiences, I think, that others can help nurture our development of confidence or help undermine it.) Kind of like what @Jeruba said, I think, just a little later, and wordier!
I’m also not sure that nerves/anxiety and confidence are necessarily opposites. I can feel confident in my ability to do something and yet be nervous or anxious about it all the same—maybe because it’s something I really care about, or it’s something I feel is critical to do/perform/execute as well as possible (with little to no margin of error), or it’s just not something I do often enough for it to feel normal, etc.
In what ways do you feel you over-prepare? Can you make some changes to your preparation process the next time you are teaching something, to see what happens?
This is just a thought I had: It might be less about confidence, and more that over-preparing and feeling-anxious have become ingrained parts of your preparation process, if that makes sense? Almost like you know you’ll “do a great job” if these two elements are a part of your process? Maybe if you make some small, calculated experiments in how you prepare, you can start to (gradually) shift your preparation process to something that isn’t so emotionally draining/taxing?