I'm sorry you had a rough test experience.
If you did not start taking practice tests until 1 week before, then I can pretty much guarantee you that you had significant timing issues and also likely significant mental fatigue issues.
First, timing: *everyone* has timing problems. Timing is always a struggle. It takes weeks and possibly months to get good at the timing. (Note: you can finish a section on time and still have *severe* timing problems.)
How to know what your timing issues are? Use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Then come back here and tell us the results of your analysis. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
The above also covers our other major issue here: I need detailed data on your strengths and weaknesses in order to advise you about a study plan. :)
Oh, wait, important note: did you take your practice tests under 100% official conditions? If not, take a new practice test now under official conditions (essay, IR, two 8-minute breaks) and then do the analysis described above. For tests taken under non-official conditions (which includes skipping the essay or IR section), your QV score might have been artificially inflated.
Please also let me know what materials / resources you have.
Re: mental fatigue, you may not have been aware you were experiencing it. Mental fatigue is very different from physical fatigue; you can be full of adrenaline and still very mentally fatigued. Read this:
http://www.manhattangmat.com/blog/index ... you-crazy/Did you feel any of that during the test, especially during verbal?
Finally, you mention wanting to take the test by the end of August or early September (6 to 8 weeks from now) and your target is 740, which would be an improvement of 160 points. That is an extremely ambitious target for that timeframe. Most people would not be able to achieve that kind of improvement in that timeframe.
I'm not saying that you shouldn't go for it, but I do want you to be aware of this, and to think about what tradeoffs you *might* need to make in future. You don't need to decide anything now but, a month from now, if you're not closing in on 740, what would you do?
Would you rather lower your goal score or take my time for the test? How would those decisions affect your admissions plan?
Broadly, you could say "My number one priority is this particular school and for that I need a really high score, so even if I have to postpone school for a year, I'm going for the score."
Alternatively, you might say "My number one priority is to apply this year so that I can start next year, and the specific school is less important. So I'm going to go do a little research into programs that don't require such high scores and apply to those schools instead."
(Note: by the way, I have no idea what INSEAD Singapore wants to see, so I'm taking your word for it that you need to score well over 700. If you're not absolutely sure - as in, hard data from the school itself, as opposed to what you might have heard random people saying online - then do the research.)
Oh, and here's another alternative - there are often differences in requirements for full-time vs. part-time (evening / week-end / executive) programs. The part-time and exec programs tend not to place as much emphasis on the GMAT, so maybe you decide, "I do want INSEAD and I also want this year, but I'm willing to go for a different program instead."