dee,
best of luck. i know exactly how you feel. I'm a substitute teacher, and on some days i dread going to work because i'm not sure which "me" will show up, the confident,happy me or the ditzy, confused, spacy me.
one thing that helps me is venting things on this support group board before i go to work. it helps me identify what i'm feeling and why i'm feeling it. kinda like a little CBT (cognitive behavioral therapy :p).
also, i try to be as prepared as possible. sometimes this is hard because i may not know what the day and students hold in store for me as substitutes must be flexible and adapt to whatever class they're in.
but one thing that keeps me sane, is something Lucinda Bassett says in her Anxiety/depression program "face the fear and feel it anyway."
I guess the main point here is to accept that you're anxious and fearful, but just go ahead and stick it out. usually the worst part is the anticipatory feelings that accompany a stressful situation. And then if things get stressful, hey, just float through it and get through the day. as long as you don't get threatened at gunpoint or anything, i think this is a reasonable goal.
Then at the end of the day, give yourself credit for getting through the day, and learn from your experience.
Well, i hope this helps you, and i'm sure you'll do just fine on your new job. I think that people like us who worry do so because we're conscientious and analytical people (so the talent is definitely there!)
Kevin