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My plan after moving out was to be home when she got here and tell her. I was going to have a friend drive me and be ready to drive me away. Am I still being too nice? ...
Don't agonize too much over doing the "right" thing when ending it, or over ending it the "right" way. Do what you have to do to protect yourself and give yourself the best path forward.
If your soon-to-be-ex is BPD, they will not respect you any more if you try to let them down easy, or behave more honorably, or whatever. Take it from me, they will bad mouth you & lie about you the same way if you disappear in the middle of the night, or end it via a face-to-face discussion.