To be honest, the fact that you're worried suggests that you will drive so carefully this time you can't fail. Just make sure nerves don't get the better of you on the day.
Practice, remind yourself you are good, ask for a mock exam.
Eat a good breakfast, avoid dairy if you feel sick. Something slow releasing carby - rolled oats, porridge, wholewheat toast, will stop you feeling shaky.
Good luck. They're scary but you can do it!