My girlfriend (I'll call her K) and I have been dating for 3 years but have been friends since we were 14. When we were in college (I was abroad and she was in our hometown) we made a new friend (I'll be calling him B). We were about 19-20 years old. B and I became fast friends and all three of us were super close until, B and K had a falling out. The incidents which caused this were

1. (2017) B and I were invited to a party by my cousin who was also a mutual friend of B and K. My cousin had not invited K. B and I initially thought it's best if we don't tell K bec we felt it might hurt her as all 3 of us and my cousin used to hang out a lot (my cousin and K are childhood friends too). I had simply told K that I was busy the day of the party and couldn't meet. B apparently told a very elaborate lie about what he did instead that day. After the party I felt horrible about lying to a close friend and I told B I want to come clean. We did and K forgive me, but was quite mad at B for lying.

2. (2017 3 months after) All 3 of had travelled to another city in my country for a concert along with some other friends and while we were there, drunk B texted K as all of us were hanging out that K was acting very pricey about something all of us were talking about, the text was actually meant for me but he sent to her.

These two incidents basically ruined their friendship and the only reason they continued hanging out was due to me and the rest of our friend group.

During this time B made 2 attempts to apologise and resolve it with K but she wasn't really interested in it.

And in all these years there was a silent cold war between them, from B's side because she never forgave him.

From K's side she said that she could never trust someone like that again, and she said that whenever I left my country for college they would not meet and B had tried to side line her and bad mouth her (which he denies doing, and K believes he only pretends to be the good guy in front of me)

I returned for good in 2019 and all 3 of us and the rest of our friends would still hang out a lot, but again B and K would not really interact.

K and I started dating 3 years ago, the mutual friends group had dissolved by then due to unfortunate stuff happening so we didn't really hang out and B also went away for 15 months.

My friendship with B has caused a lot of friction for me with both B and K. K has agreed to be cordial to him if we all meet, but mostly I have to meet him one on one or with a couple of our other buddies. This is in stark contrast to my other best friend and his fiance where we all hang out.

This is becoming an issue for me as I've had to choose between them multiple times (I've not invited him to parties at my house which he normally would have been). My parents and sister were also upset to see mine and B's friendship deteriorate as they were also close to him in these years.

I really want K and B to sort out their differences, B would be ready to talk but K refuses to even do that. I feel like K as my partner should not want my friendship to die this slow death, especially because it was so long ago and while I don't expect them to best friends or close ever I do want her to get over her hate for him. For me her not doing this hurts because I feel she would expect me to start cutting out other people in my life in the future too. I also feel that she's being immature.

I love K very much, but I'm stuck in this situation, I would choose her over him but it hurts that she would put me in such a place. I definitely would never ask her to do the same ever.

I'm planning on proposing soon but her not being a bigger person is giving me second thoughts about this. AITA here for expecting her to do this for me?