I'm sorry that you're struggling with this test. My guess is that you've learned a lot...but that you're approaching the test in the wrong way. Usually, people start out trying to take this test the way they took tests in school - that is, they try to answer everything correctly.
As they learn more, they see more and more on the test that they think they should know how to answer now, and so they spend too long on certain questions, mess up their timing, and (seemingly paradoxically) their score drops.
The problem: the GMAT test writers aren't interested in having you answer everything correctly. In fact, they've specifically set up the GMAT so that this will NOT happen. If you try to take the GMAT the way you took tests in school, you will not be happy with your score.
Read this:
http://tinyurl.com/executivereasoningHow does that differ from the way you've been approaching the test so far?
Now read this:
http://tinyurl.com/2ndlevelofgmatAnd answer the same question.
Next, use the below to analyze your most recent MGMAT CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. (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!)