Hi I've been there, I don't have real suggestions for you but wanted to say good luck! Well done for the job, if you made it to this point and are so self conscious you can definitely do well.
I would try to focus on the positive and be aware that everyone makes mistakes. If you happen to have a bad moment just keep in mind that it's part of the process (I always go through a few tough weeks when in a new job, where even my dreamjob is scary, because I'm tense and I have to settle and understand the environment etc..) and it will be better once you settle in the new workplace. Don't worry about crying or zoning out in advance. Did you manage to talk about it in therapy to maybe set an emergency plan/coping skills in order to feel a bit safer and more reassured about it for when you start?
You did so well - congratulations anyway.