When you find the answer to this question let me know. I have had the same issue my whole life. I don't know how old you are, so I'm sorry if guessing high school would be an underestimation. The reason I assumed that age range is because I had these exact problems with motivation for my whole high school career, but now I'm in college and they've almost entirely gone away (and were sent directly to other areas of my life, but that's a different story). I've found that college work is more meaningful - no "busy work", and for some classes no homework at all. Not only that but also once you are working towards a degree in something, you don't have to spend much time learning about things that don't interest you.

I realize this isn't very helpful information for you now, regardless of whether you're in high school or college. But when I had classes I couldn't stand, I don't remember ever having a strategy for dealing with them. I hated every minute of it and was selectively lazy about the work, but in the end the grade was mine for the taking. If you know you're capable of doing it, you might as well just power on through. A good grade for its own sake can be pretty satisfying.