i think you feel satisfaction after, not happiness.. though happiness might come from that satisfaction

let's say i sign up for a class and halfway into it i realize i don't really want to do it. if i finish the class i'll be satisfied that i'm done with it, but not necessarily happy. if it's a class i actually like, then i'll be satisfied and happy

That feels like dwelling on the past to me. Perspective matters: I would be happy to be done with it so I could now focus on what I actually want to do. By the way, we learn what we like by trying things, some of which we won't like. But it's ok. 

You must find purpose in the process, not in the outcome, because in the long run we're all dead