Sorry to hear that the GMAT has been a struggle lately, Henry!
So there are a bunch of reasons why you might be feeling fatigued toward the end of the test. Maybe you aren't eating a good diet both before and during the test? Are you CONSISTENTLY getting enough sleep? Are you doing something different with your caffeine intake that could affect your performance on the test? I meet tons of unbelievably hard-working GMAT test-takers who study like crazy... and then their brains turn to mush because they're working like crazy and never get enough sleep or good nutrition. I have no idea if this is happening to you, but it sounds like it's possible.
On test day, I think
it's important to keep your blood sugar stable, so a good, healthy pre-test meal is key, and I think you're on the right track with the energy bars during your breaks. But everybody is different, so there's no magic formula. Personally, I like to be wired for sound when I take tests, so I drink caffeine like an absolute fiend during my breaks -- but that's probably not normal or healthy, and may not work for you.
The other thing I notice about your prep is that it looks like you've spent as much time with Kaplan materials as with official GMAT materials. Kaplan's techniques may be helpful for some test-takers, but it's impossible for any test-prep company to write high-quality verbal questions that mimic the real GMAT. So definitely focus on doing more official questions. It sounds like you've only used the verbal guide, but not the main
OG? If that's correct, then spend more time in the
OG, and consider purchasing the GMATPrep Question Pack or (especially!) the GMATPrep Exam Packs for your last few practice tests. Kaplan's materials may have their virtues, but I wouldn't take those results anywhere near as seriously as your GMATPrep scores.
One other thought: when you do sets of questions from, say, the
OG, don't just do a few at a time. Maybe you're already doing longer sets, but if not, try to do 25-30 questions in each sitting. I occasionally meet test-takers who habitually do 5-10 questions at a time, check the answers, and then do 5-10 more -- and that makes life much more difficult on a (nearly) four-hour exam. And if you think you're ready for a challenge on CR and RC, consider using some official LSATs -- they'll feel a lot harder, but that can only be good for your stamina on the verbal section.
And take care of yourself physically! Maybe a few good nights' sleep will at least help with the fatigue.
Good luck with everything!