There was a time when I used to be equally anxious or tensed about any exam I was going to attempt. This obviously wastes all the preparation and impairs spontaneity at the exam center. You should notice particularly that when you study, you are actually imbibing certain skills, for example, when you look at a quant problem, the statements and numbers are automatically making your brain to derive a possible equation or at least provoke it to try and think differently. These are certain traits that come with sustained practise and an improper focus can ruin it all.
The most important thing at the exam center is to stay in the groove, stay in tune with the process by which your brain functions and derives the official answers. So keep solving questions, an error-log is what that rescues you the best at this time. If there's not one, then keep solving questions and keep focusing on errors. Prefer more newer questions that you haven't tried earlier.
More importantly, learn and keep reminding yourself, the fact that GMAT is a test of a series of questions that are bombarded at you and you will do your best to solve each one of them. Just guestions and your solutions, there is nothing called a score, what you do is just read the question and answer it from all that you have learnt and practised. As I said, solve and not make an educated guess at each one of them, may be, one or two is fine but when you notice you are not doing it the right away and just making guesses based on your gut feeling stuff, take a momentary break to re-focus better.
Before the exam, learn to take deep breathes and release all the air from the lungs in a whiff!!!! Do that in a solitary state, away from anything else, think only about your breathing, count the breathes you take. Do this for 3 sets of 15 each. You can clearly see that your heart stops pounding harder.
GMAT is a wonderful challenge of focus, speed, agility and composure of the brain. Let nothing disturb you while you are trying to answer the questions. Why in the world should anything else ever matter? Wouldn't you be bothered if you were spending a quite evening with your spouse/fiancee at a beautiful, secluded place and there is disturbance around. Consider attempting the GMAT exam as a wonderful opportunity with a loved one, you have infact respected it, worked hard so that you could feel good with it. Why ruin all that when you get a wonderful opportunity to be with something that you have yearned for? Enjoy every moment, cherish every question and your approach to get the answer. Make it fast and make it wonderful!!!!
All the very best for your attempt, keep us posted.