Just browsing back at these forums and saw your post. My scores were similar, 640 (Q44/low verbal) after self study, 670 (Q42/V40) after Princeton, and 740 (Q49/V41) after Manhattan GMAT
and a 2 year hiatus from thinking about GMAT and MBA.
What I found was that I could study traditionally for the verbal & improve my score substantially by familiarizing myself with more grammar rules, idioms, and critical reasoning question types & answers. However, I had a difficult time breaking out of the low 40's on Quant. I couldn't master all the math subjects completely, and even the math I knew killed me when presented in a hard question format. I was having difficulty even completing the math section.
However, I learned that the GMAT math rewards you more than it punishes you. You will score much better if you ace the middle-range difficulty stuff since the algorithm will kick your score up real high. Then even if you miss a few killer questions, you'll still be in a good score range. IF you've read enough of the practice q & a's, you should be able to eliminate your way to 50/50 accuracy on the killer stuff (there are obvious wrong answers). BTW, when you're scoring around Q50, questions at the end will be so hard you have to guess and move to save time.
Thus, when practicing, focus and nail ALL of your 600-700 level questions (MGMAT terminology for mid-upper level q's). Once you consistently answer those correctly and even if you never get consistent at the killer stuff, you'll likely score in the mid-upper 40's. On practice quizzes and exams, I was never confident on the killer combinatorics, probability, data suff word problems...etc but I was 75-90% confident in my 600-700 range questions and 99% for anything easier. I was probably <25% confident on my 700-800 level questions.
For people who score in our range in Quant and have trouble busting out, I'd recommend allocating practice Q time like this (with MGMAT questions):
700-800 level: 5%
600-700 level: 60%
500-600 level: 30%
400-500 level: 5%
Good luck and don't give up! You know it's not impossible.