((((((((((( Tishie ))))))))))))
I can certainly understand your frustration with this situation. I think it happens more than you know out in this world. There can be one of any multitude of reasons why she does this and I think trying to figure out the exact reason would be self defeating.
I believe the best thing you can do for yourself and for her is to set your boundaries and to remind her that no matter what illness she has, she is still responsible for her behaviors. Let her know exactly what your boundaries are and give her the chance to accept them and work on her behaviors or to say that she can't help herself and walk away from the relationship.

sabby