I am sorry you are in a situation where you see him regularly (can't avoid him unless you drop your class). Having time away from someone (I suggest more than a week), can put your feelings in perspective. Obsession has a "chemical" component, like an OCD habit. Perhaps you "know" it isn't right but are just caught up in it because you have few other relationships? When you complete your class, I think that then you could really get the space you need to help you stop obsessing. Sometimes we can get in these loops about certain people in our mind and much of it is simply all in our mind. If that makes sense? I am speaking from personal experience so it might not apply. I hope it helps. If not then here's some hugs: