just bombed the GMAT... what should I do?

Sorry to hear about a frustrating experience!

You know, based on what you described with the 18-month study process, the perceived burnout and sleepless night beforehand, the fatigue noticeably setting in during the test, etc., I'd recommend that you take at least the first week of your vacation (or, honestly, all of it) to just get away from the GMAT altogether. If that one week away is enough to make you forget all you've learned over 18 months, you were in a world of hurt to begin with because you didn't know it anyway, so don't worry too much about losing momentum with that - mental health and a clear mind should be top priorities.

And think of it this way - you had a rough couple weeks leading up to it, a bad night's sleep and some tough circumstances surrounding it, and you still scored 580 - significantly above average! Don't overstress this - take your vacation (or at least the first half of it) to decompress a little bit and then when you do get back to it try to think more about *how* and *why* than about *what*. It also sounds like you're focusing more on time-spent and quantity-done (you mentioned your 18-months of study and the items that you completed for study, but not much about the mistakes you were making or the conceptual areas that you found troubling) than on how you think and how you can improve. Once you're back to it, pay attention to the kinds of traps you're falling into and the thought processes that are working for you and you can definitely make some significant gains.