Well, part of it is going to be just getting in there and doing it, and seeing yourself do it, and gradually getting used to the idea that this is something you can handle. It's normal to feel jittery before taking on some big new responsibilities and challenges.
The secret to handling most fears is no secret at all, really -- you just have to get in there and do it, get experience doing it, and develop some level of mastery. It's a process, and it takes a while. You will make mistakes, but that's part of the learning process, as you know. You have to give yourself "learning time" -- time to screw up, while you're learning. Try to have good people in place that you can ask for advice, bounce ideas off of, etc.
I know what you mean about the self-concept/mirror thing. It's sort of like you're shedding one identity for another, bigger one. You seem like a confident, energetic, and ambitious guy, so I imagine you'll do fine. That's half the battle, you know -- just wanting it. That's why most people do not "succeed," imo (I'm speaking of success in conventional terms, i.e., position + income + family) -- they just don't want it badly enough. You seem to want it quite a bit, and I think that's a big factor.
Good luck to you.
