Invest your time in planning and effective studying rather than in worrying. Put your grade in perspective. Your grade is not necessarily a reflection of your preparation. Find out what you need to do better. Develop reasonable expectations.
Unrealistic goals can only frustrate and become a good excuse to give up.
Take care of your basic needs. Your overall wellness does affect your performance. Don't abuse yourself. Once you feel you know what you need to know, quit studying and do something healthy to relax. Learn when you need to stop.