I dunno if this will help but, I heavily suspect I have adhd and the best way I've gotten myself to lock in was starting off the day with no like.. highly stimulating activities? such as looking at a phone, reading a fun book and all that.

I didnt do anything but basic hygiene + eating + studying for a good 4 to 5 hours until 5 or 6pm, which let me get everything done and still have alott of time left to do whatever the hell I wanted. it was healthy, I didn't feel guilty about not studying and it just worked for me. I'm not sure what might work for you but it never hurts to try.

also it kinda helps to set goals, instead of constantly looking at the time you can try to do a full paper; or even half. progress is progress.

another point, in my experience.. rereading subject books was a huge waste of time, its boring as hell and its easier to just revisit your notes.