Too late to be too shocked

My first question to you is when are you planning to apply? If you are applying next year and this is your first attempt, I would say be ready to take the test one more time. Generally speaking, a massive jump from GMAT prep tests taken a day or two earlier from the D day is VERY difficult. And we are talking about ~300 points here. I'd loved to be proven wrong though. You said you were scoring 600-620 in the run up and that tells me that this attempt might just be a very bad day, and hence, not representative of your true potential.

So lets get down to what you should do on this test. You still have a few days left in your prep, in which you can make significant strides towards your target score.

1.) Write a couple of tests (with AWA and IR) and nail your timing strategy.
2.) Revisit your mistakes and error logs. Concentrate on your strengths and areas where you can improve significantly. Do 10-15 qs together and try to build stamina. Always, read the official explanation and try to see what is the concept you are lacking.
3.) Practice official problems only. Revise OGs, supplements, question packs and GMAT prep questions extensively.
4.) Be confident that you will be able to get what you are aiming for. Unless your mind believes it will be very hard for you.
5.) Plan your answering strategy on the test. Don't waste too much time on one question.

If there is something specific that troubles you, post in this thread or PM people (including me).

Good luck and I hope you nail it.