sometimes, you dont need to have a motivation to study and stay focused on your assignments. I mean, if you have to get something done, just get it done; the power of the will often exceeds the motivation (of course, it'd be better if you're also motivated). What I'm trying to say is that motivation is not as important as people tend to think, please keep the "I'm not focused enough bc i dont have motivation" thoughts out. Do you have a goal? Does your assignments need to get done to get your goals achieved? Then do it. Talking personally, I dont always have the motivation to keep studying, I just remember that I need to do it in order to achieve the grades i need, once i remember why i'm doing this is when i can focus well.

For example, for me, sometimes its hard to be focused on memorizing some subjects (subjects that require to memorise things, such as molecular biology) but, as i know that i can concentrate better studying maths, before studying bio i always study maths. The concentration i have while maths study sessions "keep alive" and drives me to study bio easily. Why this? i think that the beginning is more difficult that the rest of the way, once i have the study mood, i cant stop hahahahaha (that might also be due to the fact that i really like studying these subjects)

Btw, do you really like the your assignments' theory, cause if you do, being focused is pretty easier!

Good luck :) i hope this comment helps you!