Honestly relax, just like a driving lesson except it could be your last! I just passed my driving test today, second time, both times I was relaxed as anything. If you think you fail, don't worry about it, first test I stalled on a hill start and knew I failed. I stalled today pretty much straight away, but just got on with my driving, I wasn't sure if I had passed or failed at the end of the test, but I was delighted when I had found I had passed.
I 100% agree that telling someone to relax is alot easier then the person actually being relaxed, anyways if you don't feel ready when it comes to your test, delay it!