Hey Nona,
Had you taken practice tests? The amount of study is slightly irrelevant if you never took any practice tests (so you don't know where you started). Also, you say you studied for 3 months, but studying for 3 months without taking any practice tests is a very bad idea.
Of course, it's possible you did take practice tests, but then I'm not sure why you were shocked by the 540 (you didn't say in your posting that you'd been scoring above that in your practice tests).
All that being said, it isn't possible to correctly identify 90% of questions, and know the strategy for 80%, and still get a 540. It could be that you're incorrectly strategizing, in which case that's something you could focus on.
If you have a practice test with MGMAT, I could look at it and give more specific advice. Taking a first test on our site is free, so maybe do that, then let me know (you can give me your full name by PM, if you'd like, though I'll post my thoughts as part of this thread).
Good luck!
-t
Tommy Wallach, Company Expert
ManhattanGMAT
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