Hello, I feel your pain and was so very nervous to take my driving test too. It took me many attempts to build up to it!
Much like the advice already given here, have a relaxing bath the night before, try taking kalms/rescue remedy. Take deep breaths, do some stretches and most importantly be easy on yourself.
Remember, if you don't pass this time, you can always book a test again. It's all experience and at least the next time you will know what to expect.
Don't forget the test will last for only 40mins, this will seem like nothing!
During my test I thought I had failed within 20 mins which in a way helped as I had lost my nerves thinking that nothing really matters any more! Turns out I did pass and I was just being paranoid!
The examiners aren't out to get you, they want people to pass. I can remember telling mine I was very nervous and he said so was he. It takes a lot of trust to get into a car with a stranger! In essence it's just a driving test, all they want to see is that you can drive the car, perform some manoeuvres safely and can understand basic road layouts.