When is enough enough?

I'm feeling a little overwhelmed right now. I don't think I know when enough is enough. When is it ok to just say no I don't want to do this or no I don't want to do that? Today I had a few things come up. A few possibilities to do or not do something. Everytime I said no I felt bad. I felt like I was letting the ball drop somehow. I had every right to say I can't babysit, I can't do that for you, I can't give you any money, I can't pick you up. I don't know why I felt bad each time. I don't know why I always feel responsible for everybody and everything. I don't know why I can't figure out when enough is enough.