I'm sorry you had a disappointing test experience. Your quant score almost certainly dropped at the end due to the timing issues - you probably didn't make four lucky guesses in a row. Did you notice before then that you were running out of time and speed up on earlier questions? If so, that likely also caused careless mistakes, in which case your score would have dropped even more.
Even with all of that, you still scored a 580 (and you were able to put the stress of quant behind you and still do well on verbal - nice job). If you can fix the timing problems, you have a very good shot of hitting at least 620, possibly higher.
It typically takes people a solid 4 to 6 weeks to fix timing problems. The good news, though, is that most people can make solid progress on timing.
Your first task is to understand exactly where you're getting hung up and why. First, read this article:
https://www.manhattangmat.com/blog/inde ... lly-tests/Are you approaching the test that way? I'm guessing that you're struggling with this mindset.
Second, use the below to analyze your MGMAT CAT (this should take you a minimum of 1 hour):
http://www.manhattangmat.com/blog/index ... ice-tests/Think about where you would change the timing decisions that you made - where should you have cut yourself off faster and guessed? Etc.
Then, read this:
http://www.manhattangmat.com/blog/index ... -the-gmat/Figure out your strengths and weaknesses as well as 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!)
Here are two resources to help you on the timing; start doing what they say right away:
http://www.manhattangmat.com/blog/index ... -to-do-it/http://www.manhattangmat.com/blog/index ... nt-part-1/