There are probably two separate things going on (both of which are contributing to the situation).
1) You have good performance for a while, pushing you to very high level questions that are actually too hard for you. You spend too much time on them, mostly get them wrong anyway (because they're too hard - that's the point!), and then you run out of time for the rest of the test.
2) As the section progresses, you become more mentally fatigued. You can't perform at quite as high of a level and you start to make more mistakes and also make bad decisions (such as spending too much time and energy on a question that you should NOT be doing!).
Read this (right now):
https://www.manhattangmat.com/blog/inde ... lly-tests/This is your actual goal on the test: to make the best decisions about how to spend your time and mental energy.
Your goal is NOT to try to get everything right. As long as you continue to try to treat the GMAT as a school test, you will continue to have the kinds of problems that you're having.
So step #1 is changing your mindset. It sounds easy and it's quite hard - you will likely need more than 3 weeks to do so.
Step #2 is to start studying accordingly - which means having pacing plans, knowing how to decide when to do a problem and when NOT to do a problem, knowing how to make educated guesses, etc.
Read this:
http://www.manhattangmat.com/blog/index ... -the-gmat/And for pacing, these two:
http://www.manhattangmat.com/blog/index ... -to-do-it/http://www.manhattangmat.com/blog/index ... nt-part-1/Then come back here and tell me how what you have been doing does NOT match what those articles say, and tell me what you need to do going forward to get the right mindset going.
Finally, use the below (plus your knowledge of the above) to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Figure out what you think you should do based on that analysis. 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!)