I took my GMAT for the first time today and got the shock of my life. I scored an unbelievable 510! Obviously something went terribly wrong but I still can't fathom as to what that was(* still reeling in shock*). My GMAT prep test results were 600(GMAT prep 1), 640 (GMAt prep 2), 680 (GMAT prep 1A) and 690 (GMAT prep 2A) (with 2-3 repeat questions on each test). My average on prep tests had a Quant 45 and Verbal 35.
On the test day, I seemed to be getting a fair set of difficult questions too and I thought I had performed well until I saw the score. I do understand that stress, anxiety and other factors can lower scores but what I got was lower than my unprepared 1st GMAT prep. (And believe me I wasn't stressed, finished all questions on time and seemed to get harder questions towards the end, on both sections)

I realize what's done is done. Could someone please advice what should be my gameplan now? I plan to take another one in the next 2 months and definitely don't want a part 2 of this day. Additionally to give you some background, I took a GMAT course here in India but that was 2 years ago(never took the exam then). This time around, I bought the Manhattan GMAT strategy guides, OG and studied by myself. Should I join one of the coaching programs now?

Thanks a lot in advance