You need to relax. You had enough practice and your practice scores look very good.
1) You need to avoid brain fatigue before the exam. Slowly try to relax and do non GMAT related activities.
2) Get plenty of sleep before the exam.
3) Also, trust your guts and don't question yourself during the exam. Many people fail to do this on the exam and end up with a score that doesn't quite reflect his or her true ability.
4) There are few sites that offer GMAT Studycards with tips and etc. Some of them have useful strategy as well. Go over them in the next 5 days.
5) Ending up with sub-700 GMAT score isn't the end of the world. If you don't score your target score now, you can always retake the exam. R1 deadline isn't the holy grail. There's R2. You have to accept this now before the exam. Otherwise, your anxiety isn't going to go away.
6) Don't take too much caffeine before the exam and take all the breaks that they give you.
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