Hi!
the first time is always nerve wrecking. if you wernt nervous you wouldn't be normal. I think you need to lower your expectations, don't make it as big as it is. basically, do not care as much. imagine it's your driving instructor sitting next to you rather than the examiner. I'm a pretty confident driver, but exams or tests always get me flustered. So I actually failed 3 times and it was the nerves that got the best of me. lastly, don't tell your friends or family your test date. That way the disappointment isn't bigger than it should be.
have a little pep talk just before. tell yourself, you've got this. you are in control of the car not the other way round. your driving instructor wouldn't send you to a test if he/she didn't think you were ready. A mistake is only as big as how you recover from it!
Good luck and I wish you all the best!