Despite this story I don’t have a BPD myself. This is pretty uncharacteristic for me. 
Also I can’t stay mad at him. 

I had a best friend that I made in a month. It was so organic. He and I had so much in common we would understand each other without talking. It felt like I found my long lost twin. 

But...as these things often do- it got complicated. We tried not flirting but he loves breaking boundaries and I love him. Soon I was in love. I can’t speak for him but I think he was too. I have looked into this and I am pretty sure I was his FP at least. 

We helped each other through a pretty rough year in both of our lives, and despite all our efforts to stop being attracted to each other - we grew more and more intimate. 

Except he had a gf. If you really want to believe in something I guess you can convince yourself of anything. I am not dumb generally but I completely overlooked the possibility that he is in a serious relationship. The moment he told me there is a gf- I stopped talking to him. Except I got to see -“frantic efforts to avoid abandonment” in real life. And since I still loved him and missed him immensely- we figured out a way to stay friends. 

We stayed friends, we would hang out, talk about our lives, problems, accomplishments, hopes for future.
I always texted him first now. He used to text me a lot. Now it was less but still we hung out, we had a lot of fun but as buddies. He would still be attracted to me physically and I to him but we never did anything inappropriate. He would leave work/ wait for me for hours and would not want to leave. But I always thought -he can’t stay too late for sake of his relationship. We kept talking about how we don’t want to ever lose each other. He kept telling me I will leave him and forget him. I kept saying -I can never forget him-he is like a brother to me. 

Then somehow his gf found out that there is more to this friendship than he told her about. And the girl he kept talking about? Yeah she is crazy about him. So came the ultimatum- her or me. I think he did the right thing and chose his relationship and I hope he is absolutely happy. 

But.. I did lose my best friend. And despite my best efforts to be strong- I did cry on the phone -when he was telling me this. I never cry he knows this. 
And he sounded broken. Maybe I am just projecting. I told him to take care of himself and I will always be his friend. I didn’t beg,explain, I just let him go because he has enough problems without me becoming one too. It was like a hostage situation and I didn’t want the hostage (my best friend) to get shot. 

He went NC and we haven’t talked a year. I texted him recently I will always love you and I will always be your friend. I am moving back home and I am dating my ex. Take care of yourself and I hope we will talk soon. Goodbye.