ADHD

Pomodoro timers, as someone else mentioned, are great. It was a lot easier to start when I could tell myself "ok, I only have to do this for fifteen minutes" -- and most of the time when that first timer went off I'd gotten into a groove and kept going anyway.

For writing assignments, I found starting with a blank page really difficult, so I'd paste in a paragraph or two of lorem ipsum or some other nonsense text and go from there. Having words on the page, even though they were irrelevant and I knew I'd have to delete them later, tricked my brain into thinking "oh, I've already started this; carry on, then!"

The habit of doing even just a little bit every day was more sustainable and more effective for me than infrequent big cram/study/writing sessions.

I had probably the most success with using musical cueing. Whenever I sat down to write or study, I listened to one of the same two albums on repeat. (I like classical; one was Handel's Messiah and the other was a two-disc set of Thomas Tallis choral works.) Even now, almost a decade later, when I put either of those on it instantly snaps me into work mode. Creating that kind of association is really helpful! And having only one or two choices also meant I couldn't distract myself by trying to figure out what to listen to: it was A or B, end of.

You might also consider how/whether you could spread out your course load. Most years in undergrad I did five classes a semester, but one or twice I did four only (plus a summer class to stay on track) and being at only 80% of a full load made a huge difference.

Good luck!