Do you have a hard time saying no to extra work at your workplace?

I'm working part-time and going to school part-time and trying to slowly transition my son into the world of daycare.

I work as an adjunct instructor at a local college. My employer just asked me to take on another course this semester. This cuts into my studying time quite a bit, or cuts into the time I can spend with my son (right now I can pick him up at 2:00 from daycare). It wouldn't be a financial benefit to me because our hours are capped so I would lose hours next semester. So I told them no, I can't do it. I feel bad though! Even though there's literally nothing in it for me. I got passed over for a full-time position this spring so I don't even feel like I owe it to them. Plus now that I'm transitioning to teaching secondary I feel like I owe it to myself to focus on my grad classes, since that is where my future lies anyway.

I'm feeling crappy either way!