Try to focus less on your fear because it can only overtake you if you allow it. It's all in the way you perceive your situation. Focus on the end result (a positive end result!). Tell yourself that you have studied and have prepared as much as you can. Allow your best to be enough. Don't lie to yourself, but try your best to be positive. Imagine yourself completing your exam and feeling confident about it. If that's hard to do, practice some relaxation techniques like progressive muscle relaxation and try again.