Test anxiety is all in your head. It is natural to feel a surge of nervous energy before the test. The trick is to channel that energy productively to stay alert through a long test, instead of burning it through anxious and racing thoughts.
Try visualization exercises to remind your brain that you know exactly what the test will bring because of all the practice tests you successfully completed. Use a mindfulness technique like breathing exercises or meditation. Stay sharp by eating healthy foods and exercising regularly.