Hey CB-
Sorry, I know exactly how that feels and it's painful and confusing. One thing I discovered, as I considered my 'love' for someone who treated me like total crap, if what was I in love with anyway? Turns out it was the her I projected onto her, aided by her mirroring, in the beginning, and then the unfulfilled chase of trying to get back to that place, which is not love but can be mistaken for it. A borderline will be whomever they need to be to attach and turns out the whole thing was a fiction, so what exactly was I in love with? And why did I confuse the yearning for something I couldn't have with love? Lots of growth digging there, the gift of the relationship.
I just don't know how even with a replacement he can just forget about me and our relationship just like that.
A borderline appears to forget and move on quickly because they have to, the alternative, face all the issues and the demise of the relationship, would trigger debilitating shame, something he can't live with, so use the tools to not go there, same tools he uses to not seek help.
So if you were going to shift your focus from the past to the future just for a minute, what would that look like?