I think the phrase is
object constancy or as you put it, "out of sight, out of mind". I recall a post or article years ago mentioned a pwBPD who needed a t-shirt to sleep with and smell to remember the other.
I’m intrigued by this. The one strange thing I didn’t see coming was, when the instant discard happened and
S. was moving out, she was so emotionally wrapped up over memorabilia, tiny Mylar balloon from a birthday present, refrigerator magnets etc. one had fell on the floor and I asked if she has dropped it, she looked like a child with a lost dog. Even through her texting my friends she told them she was keeping the ring because I told her too. They asked me about it and I’m

…. I’ve never had that discussion with her, nor would I.
So… back to this. Is this a known thing with pwBPD after discard? I’m just now hearing of it but strikingly makes some things click