My first piece of advice is not to feel devastated at all. 680 is a great score. I know that you want more and that you were expecting more - but you didn't just score a 400.
Your quant score was close to (just under) your practice test scores. Your verbal score dropped more. So my first thought is that you were experiencing some mental fatigue that brought you down particularly in the last section, the verbal. People suffering from mental fatigue often have no idea (that's kind of the point - your brain is tired, so it's not thinking straight!).
This article describes some of the symptoms of mental fatigue:
https://www.manhattanprep.com/gmat/blog ... you-crazy/By the way, my rule for my students is: no practice test within 5 days of the real thing. You risk tiring yourself out - it's like running a practice marathon 2 days before the real marathon...and you did take a practice test 2 days before...so mental fatigue is looking more likely.
My next thought is: did you take the practice tests under 100%, full official conditions, including all 4 sections and the allotted two 8-min breaks per section - and no longer? Did you take all 4 sections as seriously as you did on the real exam?
If you didn't do all of the sections, or took some of them less seriously, or gave yourself longer breaks, or ate and drank during the practice tests... basically if you deviated from official test conditions, then you may have been artificially inflating your score. It's important to know if that's the case because it means that you actually shouldn't have been expecting such a high score in the first place.
There could also be aspects of nerves or anxiety that kicked in - that can hurt your performance and it can also contribute to mental fatigue, which then hurts your performance. Obviously, everyone feels more nervous on the real thing than on a practice test, but it can affect some people more. If you think that could be a factor at all, try this:
https://www.manhattanprep.com/gmat/blog ... mat-score/You might also consider ordering the Enhanced Score Report. It doesn't have a ton of data, but it might give you some pointers about what happened, especially on verbal.
https://www.manhattanprep.com/gmat/blog ... re-report/What do you think about all of that?