You know you are not alone with this although I know it feels like it. It really hurts when they are abusive & cruel just because they want to be. My therapist told my wife it is a defence mechanism for her. Kind of an I'll hurt you before you hurt me or I'll hurt you worse then you did me thing. I know that doesn't make it easier to accept, espeially when they keep on with it. I think anyone that has a family member going thru this can tell you, you give & give & give until you are give out.
Hope things turn around for you, or at least level out some