I don't have much experience in abusive relationships turning out well. But I do believe anyone can change. But you said it best-they have to acknowledge that what they are doing is abusive, and be willing to seek help and do the work to change.
For now, if they are not willing to do this, it's best you keep your distance and not keep running yourself through this. I understand, believe me maybe all to well, the desire and need to be there for someone who needs you, regardless of that effect on you. But you HAVE to take care of yourself. You can only give so many suggestions and discuss what they are doing to you, so often, before you just have to cut ties.