First of all, you are right. It'll become easier as you give more exams. That's the key. Keep going!
My take-aways on dealing with mental fatigue from my prep till now:
1) Reduce mental friction during your Preparation: Work on your basics and make sure you have mastered an approach for every question type. This will help you in reducing that mental friction (which adds to the overall fatigue). What happens when you work on questions randomly is that you keep re-reading the stem or answer choices. Especially in Verbal because you are not really writing a lot unlike Quant. You need to be clear in Verbal about when you are 'looking' at the question v/s when you are 'working' on the question. This will happen by having a systemic approach and 'active thinking'. Once you make Verbal your friend you will actually feel a bit energetic to go through that last huddle.
2) Dress rehearsal/Visualization/ Stamina building- Re-create the exact test scenario as much as possible. Don't skip AWA/IR (especially in the first few mocks), eat the food you will eat during breaks, test your pacing strategy, try and wear similar clothes, keep some buffer (eg: if there is a brief check before re-entering test room), etc. Basically this is your time to experiment and find your 'ideal' routine. At the same time, don't get too rigid about it all too. Expect that something will change and there will be 'variables' every time. (In my 2nd attempt, there was a 1.5 hour delay as the server wasn't responding and they had to wait to restart it till all the current test-takers took a break. So you can't plan for that
) Also, you can visit the test centre. The less anxiety or uncertainty you have, better it is. As anxiety is a subjective thing so you have to be your own best judge on this.
3) Work on your physical health- Exercise and eat right towards the build up of the test day. This will contribute towards the 4 hour marathon. Its always better to not be coughing or sneezing during those 4 hours.
4) Utilize breaks- My way is: I eat 1 or 2 bananas with honey and black/himalayan salt ---> drink water ---> go to the loo ---> splash cold water on my face ---> try to relax my eyes ---> stretch a bit and look at the bigger picture/gain some context (briefly think what just happened and what's next). I try to finish all this in 5-6 minutes. Breaks can help you re-energize. So design your way of working through the break.
Also, do what BB said
Set up your scratch pad, avoid careless mistakes, know yourself.
Here is a good article on setting up your scratch pad:
https://www.manhattanprep.com/gmat/blog ... tch-paper/