I'll keep this short(er) since this is r/BPD not r/relationship_advice (I typed out a VERY long message and decided to delete it lmao)

But basically me (18) and my bf (17) have been dating for exactly three months now. About two months in it had to go long distance since I moved back home from Uni. But we're trying to make it work. He has BPD (I forget which type, he's told me about so many that I forgot which one was him (I'm the best bf, I know)), and has recently found a FP, something I didn't know was a thing for BPD. I'll try and keep this short.

I know the concept of a Favorite Person, I'm very new to it but I decided to look into it because I should be at least a little informed about the things my boyfriend goes through. But in all honesty, I don't like this relationship he formed with her. We'll call her Trish. Oh. And Trish is 24. A solid 7 years older than him.

At first it wasn't anything too bad, just a little jealous envious boyfriend stuff, since they spent several days in a row together. We talked and called less, and it made me sad and worried. But about 4 days in, we had a conversation. In summary, he admitted that he feels like he's been neglecting our relationship and admitted that he for a bit saw Trish as a replacement for me, but he cut that stuff off immediately. So be it.

But then nothing changed. They kept talking and hanging out every single day. I even learned that they have been cuddling sometimes, and he thought there'd be no issue in him cuddling someone 7 years older than them who they've also admitted to using as my replacement in the past. Even when we do Facetime, he spends half of it on his phone texting/snapping Trish. I can go into so much more detail but I imagine my English teacher isn't on here so I see no reason as to why anyone wants to read an essay

Is this like, normal FP behavior? I'm very new to this world, never knew anyone with BPD before. So any advice would be nice. Thanks!