I'm sorry you had such a disappointing test experience.
First, to set your mind at ease, the business schools really do just use your highest score - so your business school dreams are not dashed. Also, if you were scoring in the 600s in practice, then you are capable of doing better on the real thing. We just need to figure out why your score dropped so that we know what to do to fix the problem(s).
Read this and tell me which parts you think apply to your case:
https://www.manhattangmat.com/blog/inde ... t-wrong-2/The most common causes have to do with timing, mental stamina, and nerves / anxiety - and it sounds like at least the last one was an issue for you. If we can figure out what combination of things is in play here, then we can fix them and you can get back in there to take the test again with more confidence next time.
In addition to the above, read this:
https://www.manhattangmat.com/blog/inde ... lly-tests/Were you actually taking the test that way? Or were you still using the old school mindset, trying to get everything right? That would have contributed enormously to your anxiety - especially because that strategy doesn't work.
Next, read this:
http://www.manhattangmat.com/blog/index ... -the-gmat/Were you studying in this way? If not, you're going to next time.
After all of the above, sit down and use the below to analyze your most recent MGMAT CAT (this should take you a minimum of 1 hour):
https://www.manhattangmat.com/blog/inde ... ts-part-1/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!)