[karamazov]

How's your health? The brain fog, in particular, jumps out as something that may have other causes. In particular:

* Are you exercising?

* Sleeping well? Sleeping consistently?

* Eating well?

* Getting enough vitamins? Vitamin D is a common, easily fixed deficiency that can cause trouble concentrating; you can get your doctor to test it with a blood draw.

* Any chance you have long covid?

If you physically don't feel good on a daily basis, I would absolutely dial back your work and focus on getting in shape for, say, 2 months. 70+ hours per week clearly isn't getting you where you want, so aim for 40 and put in a hard cap at 50, and get used to the idea that some stuff won't get done. Once you're feeling better, continue keeping reasonable hours and resume studying then.

Even if everything else is fine, you might just be working too much. I think the vast majority of people would have trouble studying after a month straight of oncall and 12-hour days.