Hi,
I think this is your first time for GMAT.
I know how you feel, because that is exactly what I felt before my first exam.
I STRONGLY recommend you to go ahead with your exam now.
It's truly worth of experiecing TRUE GMAT style problems on test circumstance.
Even if you don't get your target score, you can find it to be somewhat different from what you expected before, and it will lead you to better way improving your score.
I sincerely wish you end up your first exam, but that kind of things rarely happens to us, sadly.
I'm sure nobody go on the exam with 100% confidence, so don't be afraid!
ps. the more you get nervous, the less you will score. GMAT is kind of exam that test your ability, not knowledge. So, it'll be helpful to be calm in order to show your ability fully!!