In my experience, you need to learn the signs of brain fatigue. Then, you can work up to that fatigue point. Stop and rest or change activities before returning to the prior activity. For me, getting bleary eyed or needing to reread a line told me that my brain had had enough. I would switch activities and return later.

Over time, I could stay with the same task longer and longer.

Do not try to push your limits. It will backfire.

My best to you.