For a person who seems to have no empathy and take advantage of you, you sure seem attatched to this woman.
For a caring person like your self, it's hard to draw a "goodbye" line, especially since you are back in the same area again. You want to give her the benefit of the doubt, even while she is abusive. She hasn't done anything to show that she genuinely cares about you, hasn't been reciprocating. She focuses on her own pain and turns you away when you could clearly use a hand up, and she's got room to spare. She sounds selfish, and uses her schooling and psychology jargon to put you down. It's hard to let go of someone that has been in your life for so long, even when they are no good for you. I think you are doing the right thing for your self by staying away from her. Look out for number one right now.
There's just no way to make some people see how their treatment of you has affected things until they start to get the cold shoulder when they come back for more. Even then, they are feeling remorse for what they have lost, not what they have done wrong. Some people do not have the ability to feel for others. If you go back and try to make nice, she'll have an in with you again. The cycle is stopped when you stop letting her in and she realizes she can no longer get what she wants from you. Good for you for standing up for your self. You don't have to feel guilty for doing what is best for you.