It isn't uncommon to perform more poorly than expected on the official GMAT. It must actually be a very normal thing, since the online forums are full of posts from people who have experienced even 100-point drops, from simulation to official exam.
There are a number of known factors related to this issue, and without going into it too far, you should of course avoid too little sleep the night before, and too much caffeine on the day of. But beyond the obvious, there are two deeper causes for test-day panic.
1. Score Focus.
As difficult as it may be to reach your target score, it is doubly difficult to do it without focusing on the test itself. So many students so deeply want to reach a target score, that it seems as if half of their GMAT prep time is spent thinking of the score, the score-in-itself. And it may be that dreaming of 700 is enough to bring 700. It surely is enough to sustain you in your studies. But I suspect it is 180 degrees counterproductive to confuse the thought of 700 for the means of getting 700. As a first step, arrive calmly at this realisation: any time you spend thinking of 700 is time not spent getting to 700. Specifically, you are in danger of looking directly at a GMAT question while actually thinking on your dream score. This has probably happened to you many times while reading some book, and the effect on reading comprehension is generally negative. Bring your mind to that, and make-up your mind about it.
The second subtle cause of surprisingly rotten results is panic. And fortunately the root cause of panic is a lack of milestones. I believe very few of us experience any panic feeling at all while tying our shoelaces. The reason is not that tying shoelaces is simple. Try tying someone else's shoelaces, and see. It isn't simple, but by age of ten you have a stable procedure for doing your own laces. So therefore you can just go through the motions, step by step, and by now so smoothly that you do it while day-dreaming.
That will never happen on the GMAT, since it will always pay in points to give it the absolute maximum of your attention. But you can avoid panic by introducing milestones. For each of the 5 question types, you might develop a set of milestones to tell you how far you have gone, and which way you should take next. Once you've got the markers, you'll need to get in the habit of noticing them. That way you have something productive to think about, instead of the deep, dark thought ocean, in which formerly the only shiny light was coming from your target score.