how can one STOP being a co-dependent without feeling guilty?

when you have that type of personality people who are "users" seem to gravitate toward you, knowing that they can play on your weeknesses. It gets to the point many times when you are not even able to take care of yourself or your own responsibilities because there are so many phone calls or knocks at the door! honestly... is there a way to STOP allowing people to use you as their own personal "Mother Theresa?" (after all, even Mother Theresa had the entire world helping her help others..)