Don't panic! Pay more attention to the GMAT Prep tests than other practice tests as you get closer to the exam. In my opinion, the best thing about the MPrep, Veritas, etc. tests is that many of them give you access to really great review tools, while GMATPrep gives you closer to nothing. That's offset by the fact that, even though our practice tests are pretty accurate, we can't make them 100% 'true to life' as far as difficulty, content balance, etc. with the limited info the GMAC gives us. So, I usually tell my students this:
- Take the MPrep tests early (typically every ~3 weeks while you're tutoring or taking a class), and review them in detail
- Take two or three GMATPrep tests in total, specifically making the last or second-to-last test before the official GMAT a GMATPrep test, to get the absolute best sense of what score you'll actually get.
I don't think you should necessarily take another practice test before your exam, either, if you haven't already. Unless you really need a confidence booster, it'll do you more good to take it easy and get the score you're going to get. You won't have time to really internalize anything you'd learn from another test at this point...