Hi Uncle rico,
It is often said that change in a BPD never happens until there is a major disruption in their lifestyle. That while ultimatums in day to day living are bad, almost no change happens without a significant disruption. If presented with an ultimatum does the light ever come on and steps taken toward rebuilding trust
I understand your logic, everyone's rock bottom is different, a major disruption would be one way, I think that it also depends on how self aware someone is, does she feel like there is something that is off with her, how motivated is she to getting help for herself? I don't think that it's a good idea to give a pwBPD an ultimatum, it's not a good idea to give one to non's either, the person is probably going to feel resentment and possibly lash out.
At the center of the disorder is abandonment, abandonment fears, the core wound of abandonment, if you tell her that if she doesn't shape up and you'll divorce her, you may very well trigger what a pwBPD fear most, abandonment, my advice is that it's not worth the risk, a boundary is not something that you set on someone else, a boundary is something that you set on you, if she does X, I respond with Y.
I could continue the marriage if she wanted to get help (I don't even need her to change), but I can't live in a marriage with secrets especially with the infidelity.
If you're in a r/s with these dynamics, you have to accept that there is very possibility that she may never change, this is who she is, I could of written your post a few years ago, I thought that change came from someone else and I wanted my ex wife to change. Nothing changes without change, change comes from you, when you change, it has a domino effect, everything changes. What is your boundary, is it a deal breaker for you with infidelity or is it something that you think that you can work through in counseling? You can repair a r/s if there has been infidelity, it takes a lot of hard work.
Here is an article, a tool that is used in DBT for pwBPD but anyone can use it, it's called radical acceptance, that's another option, you can radically
accept her for who she is, I'm not condoning infidelity, everyone is different and has a different opinion about it, it depends on our core values if it's something that we can or can't work with. I hope that helps.
Radical Acceptance For Family Members (DBT skill)
