I Am Too Nice

Not to toot my own horn, but...I am too nice!  I am nice to the point that I irritate myself.  Sure, I can be rude and snarky, sarcastic and stubborn, and even self-righteous and judgmental.  But, most of the time - at least to people's faces - I am just too nice.
I know that I need help sometimes and that's part of why I feel I need to give it when I can.  There are plenty of people I am perfectly happy to help in any way I can.  But there are others who seem like they would be better off if they were forced to get by on their own so they can learn from it, yet I'm still their crutch.  As the crutch, I am preventing them from learning to be self-sufficient, mature, and responsible...or at least this is what I tell myself when trying (and failing) to convince myself to put my foot down.
Anyway, the point is it bugs me that I can't tell people "no" and let people take advantage of my kindness.  All the while I'm "helping" them, I'm silently asking myself questions. "Why won't they leave me alone?" "Why can't they bother someone else?" "Why can't I tell them to just go away?"
I think the reason is that I'm afraid they'll think I'm a bad person.  I'm afraid that they will blame me for their continued misfortune and that I'll blame myself as well.  I'm afraid to make them mad or upset them.  I'm afraid of what they'll think and afraid to make myself feel like a bad person.  I know that it is normal and healthy to be able to set boundaries, but I simply can't do it.