Every single time a girl talks to my FP I become insanely jealous and upset. It hurts knowing they are talking and there is absolutely nothing I can do about it.

I have no romantic feelings or sexual feelings towards him, we are simply just best friends and have been for years. He understands my BPD and has explicitly said to me countless times that we are best friends, he isnt going anywhere and if a woman were to demand he stay away from me then he'd drop her. We've been through a lot together and I dont want to get into it all but we've always supported each other.

There is one specific girl he keeps talking to and flirting with, although shes asexual she just casually plays along and he knows it doesnt mean anything. Regardless it hurts so much. He says he enjoys flirting with her because it makes him feel wanted and that its unfair of me to go flirt with other women but he isnt allowed to. 

Its getting to the point now where Im sick and tired of feeling this way. I just want it to stop. Everytime we hang out its great and Im really happy up until she messages him or something and it just hurts.

Ive tried mindfulness techniques like the conveyer belt and noticing the emotions but not acting on them. But the thing is I dont know where these emotions are coming from and thats whats halting my progress.

Im good friends with this girl too and if she knew it was hurting me then I know she'd be upset about it. I know there's nothing malicious in what she is doing, shes just having fun and hanging out with her friends.

I dont understand why Im so hurt and jealous. It gets so much that I cant hold it together and I end up just wanting to avoid her or my FP altogether. But  i really enjoy my FPs company. I dont know what to do.

Why do I feel so sad and jealous?