I have really been struggling with this lately myself. You have probably tried these things already but this is what I have been doing, perhaps there might be something that will help:
I have been trying to study in shorter increments with some more breaks. It has helped to eat better, increase my exercise and by using some brief walks for some of the breaks, doing something fun in between as well for a complete break from it, and then coming back to it. The exercise also helped with being more alert, more than the caffeine, and in sleeping better. I've started to use different ways of varying the studying from how I'd normally do it, using different technology, walking around while I read my notes and books, reading out loud, writing it out or typing notes, etc. Just to change it up. Before I read, formulating questions that I want to get answers to so it gets me more engaged has helped as well.

I noticed too that a bit of depression, stress, and anxiety was creeping in. I was already on meds for this, so I need to get them adjusted. Not sure if this is a factor for you or not, but if so, something to consider as all this can influence concentration level as well.

Wishing you the best luck in your studies!!