Mental exhaustion is common in graduate [gradual students, we used to say] work. I know of which I speak, as I have 3 degrees [fool that I am], though I didn't know I had MS at the time. My dx came much later, but the damage had been around for at least 30 years [since my teens] when I was dxd in my 40s.

I'm so glad I didn't know back then===maybe I would have thrown in the towel and left grad school!

I did survive, as you will====take the weekends to totally rest, work like hell the rest of the time! I survived on lots of coffee and was much slimmer then - And best of luck!