I used to have a friend, we hooked up a lot. We were also confidants.  Both in open relationships, our boyfriends were fine with it. Over time we became close, and typical BPD symptoms started to spring up for me. I couldn't stop thinking about him, I became very worried that all the old close friendships that had rejected me or mistreated me would happen again. Of course, I romanticized friendships more than I did relationships. After all, my friends were the closest things I had to a family my whole life. But they all left me. 
One day my friend and I got coffee and a movie. He was telling me about this one hookup with this guy during a magic tournament, how they spent all night talking about life. Basically all the things that we did. That was when I started to panic the narrative of "if he's like this with complete strangers, he must not really care much about you does he?" I had such a bad panic attack after the movie. I talked to my boyfriend and I wrote down how I was feeling and then decided to bring it up with my friend. 
I thought the mature thing would be to be open and honest about how I felt. Honestly, maybe I was looking for validation, him telling me I was stupid that the voice in my head was untrue. When I called him, telling him I had these anxieties, that it wasn't his fault and that I had no intention to police his actions I just wanted him to know, etc etc. he told me what a "turn off" it was. 

Our friendship kind of devolved into dysfunction and ruin after that. Thankfully it got me diagnosed eventually, but the event has always played in my head since. Was I being so horrible? Is it all my fault? Should I have never told him the anxieties I had? If not, then were we really such close friends in the first place? 

I'm very through with romanticizing friendships, it's toxic. I just wanted the feeling of a family... 
