Don't cancel it! Trust me that you just need to go for it and see what happens. I already failed due to always stressing over the fact that I'm sitting next to an examiner, but to be honest you just need to tell yourself "it's just another driving lesson" and carry on doing everything that you've been doing on your driving lessons. Always remember that a mistake could only be a minor or even nothing, so don't feel discouraged if you stall or if you're taking your time to make a decision! - just carry on safely! Don't peek at the sheet, breathe and get yourself Rescue Remedy drops from Boots if you feel very overwhelmed with situations like this.
Also, I find that not telling anyone about your exam makes it less stressful - well at least for me. If you have told someone, try not to think what will happen if you fail. The truth is nothing will happen, you will keep your head up and carry on! Don't feel pressured to pass the first time because it doesn't matter if you pass it then or after 5 times... what matters is that you WILL get your license and that you will stay safe on the road. You will do just fine whatever the outcome will be! xx