It makes sense that having everything so intertwined (car use, insurance, address) was making life more stressful.
I think it's wise to disentangle as much as you can legally; that can take a lot of pressure off the relationship.
When BPD is involved, telling the pwBPD to do things -- especially important things, or necessary things, or things on a deadline -- often doesn't go well. For whatever reason, the pwBPD doesn't do those things, and then we get frustrated, stressed out, fearful, etc.
It is important to find ways to move forward with what needs to be done, without needing to rely on the pwBPD.
We must learn to be proactive for ourselves and do as much on our side as we can. This can decrease the tension in the relationship, and allow you to move on with your life without waiting on the pwBPD, who may struggle to execute even simple tasks.
The situation got heated because I told her I won’t consign the car anymore and we will need to transfer evento her name including car insurance. I told her I am done paying for her car insurance and done helping her with the car loan.
I agree with you that the worst possible situation is that you have full ownership and liability for the car, and she uses it recklessly.
Is she agreeable to owning the car? If so, that is good.
If not, I'm not sure you can transfer it to her without her consent.
I wonder if you can come up with a plan to end her use of your car, if she does not cooperate with the transfer.
It may be uncomfortable or a hassle, but it sounds important that she no longer use a car you legally own.
I also told her to change her address to where she lives right now. I don’t want her mail showing up at my house anymore.
If you live in the USA, you can do this on your own, without her involvement.
Go to a physical post office and tell them that that person no longer lives at your address. They may give you a form to fill out, or a form to give to the next postal worker who delivers mail to your home, or they may have some other process, but you do not need anyone else's cooperation to stop incorrect mail delivery.
It might still be wise to use skilled communication to give her a heads up: "Hi Daughter, just want you to know that as of Day/Date, your mail will be going to Central Post Office, not here" -- just as a courtesy.
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She started to threaten and I just Told her. Whatever you do is your problem I’m not to blame for you own choices.
What did she threaten -- was it suicide threats again?
Was that over text/phone call, or in person?