Hello!  Me (23m) and my partner (21m) have been best friends since middle school, dating for five years now. They were always really isolated and I was their best friend online, and they described me as their favorite person for a long time. 

This has changed. I don't have BPD afaik and anything offensive comes from ignorance, I apologize. But my partners FP seems to change. First it was this dude somewhat younger than them, and they cheated on me for months. It didn't become me again and we kept trying but... Now they have a new person. Best friend, introduced them to their newest hyperfixation/special interest.  I don't suspect cheating but...

I can't compete. They want to hang out with her so often, we don't even go out anymore. My partner gets upset when I talk about it because they feel I'm being jealous and paranoid but, I'm not.  They text their friend all the time, during dates, ditch me when their FP is free, have almost broken up with me.  They may still love me, but their FP is pushing them away from me because they think I'm abusive for being upset and wanting some time with my partner. Meanwhile I'm being gaslit so they can spend time with their FP. 

My partner is either unwilling or unable to talk about this. It makes them feel like a bad person anytime I point out something that hurts me, and then they complain to their FP who then tells them how I'm abusive and they're allowed to have friends and etc.  I just want there to be some kind of balance, and something that helps me feel important, but now that I'm not their FP they seem... bored, basically.  

So... Any tips for coping with not being your partners FP?  Or, tips for having a better conversation about this with them that might be more productive?