I'm sorry that you had a rough test day. We'll figure out what happened.
Did you take your practice tests under 100% official conditions, including the essay and IR sections, the length of breaks, etc? If you did do essay and IR, did you take them as seriously as you did on the real test day?
If you didn't, then your practice test scores might have been artificially inflated. For instance, if you skip one or both of the two earlier sections, then you'll be used to finishing the test around the time you're either starting verbal or are only halfway through verbal. That could cause a score drop when your brain runs out of energy.
Next, what are your timing issues? (Note: everybody has timing issues. The issue is just what they are and how serious they are!)
If you're not sure, use the below to analyze your most recent MGMAT CAT(s). Actually, I'd like you to do this regardless.
http://www.manhattangmat.com/blog/index ... ice-tests/Figure out what you think the major issues are based on that analysis (even though your scores were high than on the real test, the data can still tell you a lot). 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!)
Serious timing issues can cause major score fluctuations, especially when you're dealing with the stress of the real test (and especially if you are also struggling with mental fatigue!).
Was your quant score about what it had been on practice tests?
Also, read these two articles:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/Were you already aware of those things? Do you think a mindset issue might have contributed to your score drop?