Ask HN: Dealing with mental spillage

I don't actually work 15 hours a day. But I can easily spend around that time thinking about my startup, especially when I am not working on it(ie. in class; while hanging out with friends).

This is not just about startups. When I am doing something else that I really dig, I think about it all day. And if my love for that thing is greater than my startup, I can be working on my startup and day dreaming about the other thing. Because of this, when I am doing a startup I am hesitant to get involved with any activity that might make me fall in love with it(and thus pull me away from my startup)=)

Does anyone else work in this obsessive way? If so, what's your perspective on the mental spillage between going from doing something you really like to something else?