first of all nothing to panic. 3 weeks before my gmat date, I scored good in GMAT prep 1, 1 week before my gmat date, I attempted gmat prep2 and could not complete it in time. I didn't postpone my gmat appointment and reattempted my gmat prep 2 two days later, and completed it within time.
It is your own decision whether to postpone your gmat appointment. As ketan pointed, I too am a kind of person who is fueled by my defeats. And the nervousness and anxiety on the exam date actually helps me perform better by helping me keep awake and stay focused all the time. ( just so you know I daydream a lot, and daydreamed in almost all of my practice tests, do not ask about what!!)
so as said, you know yourself. Based on your past experience with gmat/other exams, you can make a decision now. good luck !
My GMAT Journey: 720 + dare = 760