Happens to me all the time. I'm autistic and it's just part of how my brain works--it gets stuck on one thing, whether that's Tetris or relativity theory, and just doesn't get unstuck until I've mastered the subject. The way I figure it, the better I manage my time, the less it hurts me when I get stuck on something that doesn't help me at school. I've given up on trying to extinguish the obsessions; they only pop up again on another subject. The thing that seems to work best for me is trying to optimize my time in other ways so that I have as much free as possible, and more chance that I will get done what needs to be done. And, anyway, even the more useless obsessions often have beneficial results; Tetris, for example, apparently increased my spatial reasoning ability and allowed me to learn to drive a car for the first time at age 25.