I hear your pain. Like you, I have been studying for GMAT since January. I sat it a week and a half ago and got 650; Q38,V41. Totally bombed the quant section - I got too excited and rushed it, finishing with some 12 minutes to spare - then had a bit of a freak out. So despite getting 710,Q46,V42 in my last GMAT prep, I didn't get anywhere near that quant on the test. I didn't really get the nerves though, it was more over excitement.
You really did well to bring your score from 600 up to 660 in little over 6 weeks. However, maybe you need to consider leaving it longer between now and the next GMAT? Giving you enough time to get your score up a bit further.
One thing that has helped me in all parts of my life with nerves/negotiation/high pressure situations etc. is to psychologically prepare for losing in all situations. I already know what I am going to do with my life if I don't go to business school - and I am happy with that option. If you always have a plan B already in place, it can make situations less daunting.
Perhaps consider seeing a sports psychologist for a session to get some strategies on dealing with nerves? You're maxing out quant at 44/45 at the moment, and your average verbal on your last 3 tests is around 39/40 (I'm ignoring your resits) - your actual GMAT is not that far off. So I don't think nerves alone is your issue, BB is not going to be a magic bullet for you. If you had said that you were sweating the whole time, that you bombed all 4 sections of the gmat, then maybe nerves is such an issue that you would need to consider BB, but that doesn't seem to be your problem.
I also did the
MGMAT review on my test scores. We identified that one of the issues was that I was not reviewing old material that I felt very comfortable with, and because I have been studying since the start of the year, it is starting to leave my short term memory. Could possibly be an issue for you also?
Both your Q and V have gone up in each test, no reason why they cannot go up again.
GL!