As our exs mirror us to fill the void in them at some point their egos and real selves came to the fore with the realisation they are "not us" in order to break with us and assert themselves is devaluation the brutal consequence of them smashing there false selves I.e us ? So they can assert their ego ?
A borderline doesn't have a fully formed self of their own; asserting ourselves as an autonomous individual comes later in our development. What we were initially was a perfect attachment, someone who would 'complete' a borderline, a fusing of psyches that reestablishes that bond they had with their mother in infancy. Mirroring is done to affect an attachment yes, but also to 'take' the good a borderline sees in us as their own, part of that completion.
But when the perfect attachment proves to be imperfect, and the fears of both abandonment and engulfment show up, a borderline gets triggered, meaning they start feeling emotions they can't soothe, and we are no longer the soother, we're the trigger, time to use the tools, the defense mechanisms, to off all the shame and self-hatred on something external, us, to feel better, and also time to look for the next 'perfect' attachment.
All of that has everything to do with the disorder and nothing to do with us; the cycle would play out the same way with anyone. Think about how sad that is: meet someone who appears perfect, mirror them to feel great, have the mirror crack, feel like hell, trash what was once seen as perfect to again feel good, repeat the cycle, never getting the pure love and wholeness that they want, just like everyone else. Sad that. Maybe time for some compassion, as we heal and grow on our way to creating lasting intimacy with someone capable of it.