Paritosh13,
Ugh. I have been where you are and it is quite possibly the cruddiest feeling. I too did well on my GMAT Prep mock tests the week leading up to the test and then ended with a 650. I retook the exam a month later and earned a 720. I opted to purchase the Enhanced Score Report and focus on my weak areas. Surprisingly, I had earned 96th percentile on Sentence Correction, which was going to be my focus for the retake; however, knowing that Reading Comprehension and Critical Reasoning were my weak spots on the first exam I was able to focus on those and earn a V41 on the retake.
What worked for me might not work for you, but it is something to consider:
1) Get the Enhanced Score Report; analyze your weak spots.
2) Take it a month later even if that means applying to round 2 - having the right score for you will be better than worrying about how to explain a score you're not confident in.
3) Focus on the questions you have already completed instead of introducing a bunch of new material. Go over your error logs and understand the traps. Clearly Quant is a strength so spend some time with that but focus on Verbal more. If you can get a sense of the types of tricks the GMAT employs; there are only so many used.
4) Don't freak yourself out as that will only make you worse. I went into the second attempt not giving a second thought to how it would turn out. I was more relaxed, thought clearly, and enjoyed the exam far more than on my first attempt. So, relax, because it's not worth stressing over.
5) Reschedule for a time you know you will be wide awake. That was part of the reason I did poorly on the first attempt, and part of the reason I did so well on the second attempt. I was groggy and anxious for the first exam, but I was awake, focussed, and sharp for the second try.
Each GMAT Warrior's road is different, but I think my suggestions will certainly help.
Best of luck,
~David