Learning when to stop takes time. Take responsibility for your actions. If you studied enough, be proud of yourself. What really matters at this point is not the potential grade, but the fact that you did what you were supposed to do. Do not abuse food before the test.
Some students use food as a way to reduce anxiety.
Do something relaxing the hour before the test. It is to late to try to learn what you did not learn before. Last minute cramming will cloud what you have learned before. It will also undermine your confidence. Then, go out of the room and use the remaining time to walk and relax.