Yep, it is exahusting especially if, like me, you're an introvert and any kind of interaction with people takes energy. On top of that, it's unlikely that you'll really see the fruits of your work -- the foundation you build with them now won't really begin to show until they build on it in higher-level classes (and maybe even projects/work after they graduate). Since I stuck around at the same university for undergrad and grad school, I've had the pleasure of running into senior students that I tutored or taught when they were freshmen and having them tell me just how much they benefited from my efforts. That's one of the best feelings in the world. So, keep at it! Things may seem futile now, but in the (very) long run they will be better people because of you.