Hi, Crimson. Congratulations. This is a positive step.
You must not entertain thoughts of backing out of this, as this was your goal. Once you get there and the ice is broken, I'm confident you'll feel less awed by it. Allow the therapist to take the lead. Don't expect overnight miracles. They don't happen like that.
For tonight, prepare by reducing your anxiety level as much as you can. A hot bath, some soothing music, a warm beverage can help.
I know this sounds gross if you haven't heard of it, but you've tasted hot chocolate made from warm milk? Try heating some milk in the microwave and stirring in a tablespoon of honey instead of the hot chocolate. Drink before going to bed. It will help you get a good sleep.
Practive hourly deep breathing you learned in the panic program. Stretch every hour. Let us know how it goes?