As the title suggests, I don't have BPD, and I used to be fairly close friends with a (quiet) borderline. I also have a family member with BPD. I decided to post this because I thought some of you might be interested in this perspective.

Being a reserved person, I generally find it hard to make friends, and tend to invest more into the few I do make. When we met, she did her best to mold her personality into one that I would like, and I quickly came to consider her a close friend. The feeling of intimacy that comes with the idealization phase of any interpersonal relationship with someone who has BPD is something that I've always lacked in life, so I enjoyed it quite a bit. We talked on a daily basis for about a year, but then she started to split on me (I never found out why). In the period when she had begun to split on me but was still in contact with me, she mentioned that she had recently been spending most of her time "lying awake at night and trying to shower", and that outwardly, she had "appeared friendly and fantastic, but was actually depressed and dead inside". I tried to ask her why, but she said she didn't want to talk about it. I decided not to press the issue, and reminded her that I was there if she wanted to talk. When she cut me out of her life, she never told me why, just that she didn't want to be friends any more. For a long time afterwards, I kept trying to reach out to her and talk things through, but she never responded. It certainly wasn't a pleasant feeling to have a close friend suddenly cut you out of their life without even telling you why (imagine if one of your friends did that to you), but I was never "lying awake at night and trying to shower" depressed. I get the sense that she really wanted things to work, but couldn't stop herself from splitting. She was pretty self aware, and even told me it wasn't my fault she wasn't responding to my messages. That made it hurt less in the moment, and it also made it both easier to forgive her and harder to move on. If she had been a raging borderline and deliberately tried to hurt me, it certainly would have been more painful in the moment, but I eventually would have come to resent her, dismissed her as a crazy bitch who doesn't deserve my affection, and largely forgotten about her. Her sadness was shorter and more intense, while mine was milder, but has stuck around for longer. I'm not actively sad over it any more, but I still get a sense of melancholy when I think about her. I've forgiven her for it, and I'd be happy to let her back into my life if she reached out to me again, but I doubt that will happen. I think people with BPD are more able to move on from situations like this because their emotions are so overwhelming that they have to in order to function, whereas with people like me, it can just stew in the back of my mind as a dull ache of sadness that gradually becomes less intense with time without me ever really needing to move on in order to function in life. 