Out of love, caring, or necessity (in the case of children) people stay in these relationships thinking that their next act of kindness or their next precious gift will make things better. It never does, and that is the sad reality of this personality type. No amount of goodness or contriteness on your part will ever get them to change.
The person that is forced to change is you, the victim, who will have to learn to either “take it” as one victim told me or to become so risk averse that you can never speak your mind nor enjoy being in the same room with this emotionally unstable personality. Why people remain in these relationships is often complex or a total mystery but one thing is certain, the unstable personality needs help. And you personally can’t fix them.