It's a long story but basically I'm in a long distance relationship and 4 months ago my SO lied to me. They were helping out a "friend" and thus she lied to me Multiple times and this completely broke my trust with them. 

Since then I have being trying to make it work and trying to get better both in myself and the relationship, things have still been very rough and a lot of ups and downs have happened. But basically because of what happened I couldn't stand the thought of my SO and this "friend" still talking. My SO other agreed and said that they wouldn't speak to them behind my back. Anyway she told me that she misses this "friend" and I can't handle it. I try my best to not let it get to me, I distance myself from the situation but then when I speak to them all they do is sit there and be upset over it and I don't know how to deal with it. The event that caused all this is very long and complicated but to some it up this "friend" was trying to get back an ex and got my SO to flirt with the "friends" new partner. Things were sexual and this happened for a long period of time and when I found out I was destroyed. After speaking to her "friend" she denied everything. I knew what had happened but she denied it all and never once stood up for my SO which surely you would as a best friend right?

So yea my SO brings up she misses this So called "friend" after everything that happened and I can't handle it. My SO is bipolar and earlier said she was going to end her life but didn't. She them broke things off because she said that all she does is cause problems etc.

I feel so stuck, there have been so many downs in this relationship but I constantly think they'll get better and perhaps for a short time they do but then things come crashing back down. I truly want things to work but I cannot stand her anymore. I can't deal with the way she feels about this "friend" who effectively helped ruin our relationship and never even apologised to my SO for it.

Any advice is very welcome, I wasn't sure what flair this fit under sorry. 