Relax and take a deep breath.
The GMAT is only an exam and the MBA is only a graduate degree. You don't really need either. Yeah, heretical advice for these forums probably, but you'll do your best if you just stop putting yourself under so much pressure that you can't think calmly and rationally.
Going by when you made the post I'm guessing your test is Monday, i.e. tomorrow. If you read this tonight, just sign off and go to sleep. Drink a beer if you have to.
Some variation (up to +/- 40 points is normal between the same standardized tests). Just think of your GPrep test as an outlier. You will do just fine on the actual test as long as you keep calm.
In the worst case you can take it again in a month. Next round of deadlines are in Jan, so even if you don't do as well as you think you should, there's time to take it again; just be reasonable about splitting time between essays and the test.
The only ability the GMAT is an indicator of...is the ability to do well on the GMAT.