Hmm, it sounds like you were going too fast, then. Was your timing like this on your practice tests as well? I asked about running out of time because that's how most people have timing problems, but it can be equally bad to go too fast, because then you make careless mistakes that you don't even know you made. And you tend to go fastest on the problems you think are easiest, so that's where you make those careless mistakes, and it is FAR more detrimental to your score to get an easier question wrong than it is to get a harder question wrong. If you're doing that, you'll just never be able to lift yourself higher because you'll constantly be undermining your score through frequent careless mistakes.
If there's literally
no difference that you can see between your practice tests and the official test - then you may need to take a class or work with a tutor to understand what's going on here. I'm sorry I don't have the "magic answer" for you as to what's going on, but right now I'm as baffled as you are! Basically, there has to be at least ONE substantive difference in order for you to perform so much lower on the official test than you do on practice tests, but - so far - we haven't figured out what that is.
I forgot to mention, if you took one of our classes or tutoring packages, you can sign up for a free Post GMAT Assessment to debrief from the official test and make a plan for taking it again - so please call 800.576.GMAT or email
studentservices@manhattangmat.com to sign up for that if you qualify. Then you can start to dig more deeply into what's going on.
Also, you said that you do get nervous and tend to underperform but that you "this time I was not." What changed so drastically that you didn't get nervous at all this time? I've actually never talked to someone who did used to get nervous but completely fixed that problem - usually, people find a way to manage it somewhat but they are still nervous and still don't perform as well as they know they can on practice tests.
To answer your question: your previous scores have absolutely no impact on a new score as far as the test / algorithm is concerned. You have a clean slate when you sit and take a new test. (Your previous scores can be indicative of a new score, though, as it's still you taking the test, of course!)