She is now madly in love with this new person after a month. They are moving in together, have talked marriage, and even got matching tattoos after 2 weeks.
Hey Belle, This "head-over-heels" type of r/s is typical for someone with BPD. Sad things is, as Tomacini notes, it never lasts because she will discover that the new person is unable to resolve her chronic emptiness, either, after which the whole cycle starts over again, either by recycling with you or finding someone else. There's a black hole of need in a person w/BPD, in my view, that no one can fill.
It's hard, I know, but suggest you be grateful that you are out of the loop.
LuckyJim