It's very often said that borderlines go through three phases in their relationships - idealize, devalue, and discard.

At first the BPD person will idealize their partner, then take their partner off the pedastal and devalue them. And finally the BPD will discard their partner.

It's pretty much the hallmark cycle of borderline love. But it can also occur in other relationships with the borderline, and in relationships with non-borderline individuals.

I stumbled upon an idea today about why borderlines discard and it made a lot of sense.

Aleyn Dowd, MA MS, on Quara states:

By far, one of the most common devalue/discard prompts is the person's refusal or inability to collude in the maintenance of the pwBPD's false self. 

From what I've seen and read about BPD relationships, this makes a lot of sense. The BPD's false self is very important to the BPD. The BPD needs continual affirmation and validation of this false self, and people who won't help support the BPDs false self just aren't much use to the BPD.

From what I've seen, BPDs are very much about the image they project and which themselves want to believe. When the truth challenges that image, and if you're the messenger of that truth, you will not be well liked by the BPD person.

It really does seem that what triggers BPD discarding is often all about whether the BPDs false self is supported by the partner. The false self might be considered the cornerstone of the BPD mind, and it makes sense that the BPD mind would want to get rid of anything or anyone that doesn't support it.