Do a mock test with your FI, or even better a different FI. I was sooo nervous before my mock skills, I think I'd almost run out of nerves by the time I reached the actual test! I was nervous before the test, but by the time I was taxiing the plane I felt OK.
You will make mistakes. If it's important, tell the instructor - I messed up my VOR position fix and told him it was rubbish and re-did it. If you point out your mistakes the examiner knows you know what you're doing. He will spot if you wander into class D for example, so rather than pretending it's not happening, tell him and tell him what you plan to do about it.
I don't think examiners try to pile on the pressure - mine "read" a magazine for most of the flight!
At the end of the test my examiner said "OK, I don't think you will kill your mother." That's what it's all about - are you safe to fly, not are you a perfect pilot.