Hi all, been lurking here since I got poly bombed and finally decided to post. Sorry for writing a novel

Me 33yom and she 31yof have been in an agreed upon monogamous relationship for a little over 5 years. Literally the best relationship of my life, an amazing & supportive partner.

A little less than a month ago when I was working I could tell she was feeling sad, we were messaging back and forth and kind of out of the blue I get a " can we please talk" moment. I end up receiving a message that starts with how would I feel about her having a second partner, my heart drops... Then I keep reading and she mentions having somone in mind, at this point I'm barely keeping it together... Then she says that person is our mutual best friend... Then I go through a roller coaster of some serious stuff. Felt things I didn't know I could feel.

Now a bit of context, this person was a work buddy and we became really good friends, my partner has a lack of friends and my friends became her friends and they get along great. I've had feelings of insecurity and jealousy regarding their friendship in the past, I saw it as a ME issue and because I trust her and believe she would not do anything to hurt me did not say anything and I got over it. Later on I would tell her that I had these feelings and worked through them and she assures me she could NEVER date that person, like NEVER.

Well we spend alot of time at his place together, we are all smokers and we can smoke inside at his place and not our own, plus were often all gaming together in the same party so it's more fun to actually hang out. I recently changed jobs and work shift work including weekends and nights and I was always fine with her spending time on her own over at his place they are friends as much as I am friends with him.

When I got home after being told she had feelings for and wanted to be in a relationship with not only my best friend but the only person that manage to rouse insecurity in me in our relationship, I told her I could not go along with it. I agreed to monogamy and will not bend on that.

My partner said she wants monogamy with me over an open relationship. If opening up means losing me then we will not open up.

I expressed alot of feelings to her, feelings of inadequacy, feelings of abandonment, fears, I was very honest and emotional during our talks.

Now we are still spending alot of time over at my friend's place, when I work super early and have to go to bed by 8:00 or 9:00 pm, she stays there and I don't know how to let her know this is causing me great distress without fucking up the friendships or without seeming like I'm being controlling.... The fact that we share the same social circle and we don't have many friends, means I'd be essentially isolating her.

I don't really know what I'm looking for here, maybe some success stories of couples surviving the poly bomb when a close friend was the potential addition.

TLDR: Girlfriend wants a relationship with our best friend and I said no. Now I'm struggling with how much time they spend together but don't want to be controlling or isolate her from her close friend when we don't have many friends.