I can only explain what I mean by this with an example or two, and the revelation I had in therapy. 

It is very hard for me when close friends of mine get into relationships. It's not that I don't want their happiness, of course I do, but it leaves me feeling unwanted, replaced, and brings up feelings of abandonment. I become resentful and cold, and it takes a lot of effort go reel in my emotions. 

I am in, and have been in, a healthy relationship for around 4 years now. It's been very hard at times, I've grown a lot, and most days I am content (in the context of the relationship). 

Which led me to question, why do I feel this way when close friends are in relationships? After multiple sessions, my therapist helped me work through my feelings and (somewhat) understand them. 

He tried to explain to me that, oftentimes, a person cannot get everything they want out of one person. What they get in a romantic partner they may not get in a good friend, and vice versa. Aside from the obvious BPD traits (feelings of abandoment, needing to be in control), I also feel that I do not *get* different things from my friends. Yes, physically the relationships may differ, but emotionally I treat them as equals which I feel I should. But maybe this intense emotional attachment to people is what makes it so hard on me? I'm honestly still trying to figure it out. If anyone else has experienced something similar I would love to hear it and discuss it further!