This is more of a Would I be the asshole as I want to avoid being an ass.

My boyfriend (M/27) and I (F/26) have been together for about 2 years and also live together. My best friend we will call her Stacy (F/25) have became really close friends due to working with each other. All three of us met because of job which I have currently left and so has my boyfriend due to recent events. However she is my best friend so we are still very close. She is like a platonic soul mate to say the least.

So my boyfriend and I hit a really hard rough patch the begining of this year. Literally the clock struck 12 on New Year's Eve and I started finding out my boyfriend has hid a lot of important things from me. I have felt very betrayed and questioned often if I even want to work on things. We sperated for two weeks and then decided we wanted to work things out and as many could imagine it has not been easy.

On New Year's Eve I just so happened to run into some things that sent me down a rabbit hole of investigating. My boyfriend did not clean his tracks and I happened upon a few things that led me to find out he had relapsed 3 months prior (not even knowing he had a history with addiction) and had also been emotionally cheating on me. I also found out that he had dated a co worker of ours we will call Emily. Emily is also younger she is 21 years old. Essentially they were never exclusive but were dating and there was some overlap that I was not aware of.

Now Emily has never done anything to me. However I feel as they both lied to my face. Emily is a very sweet girl and I always liked her and enjoyed working with her. Obviously she is way younger so we never hung out but we had a normal coworker relationship where you send memes on weekends and chat during a shift. Stacie and I literally adored her and built a co worker friendship with her.

My boyfriend never told me there was any history there. And she never brought it up as well. Which I'm upset because I feel as they both lied to me face. I feel like he should of told me that Information before I bonded with the kid. However I know it is not her responsibility at all especially considering I was the other women in his life while they were dating. She doesn't owe me anything.

So my best friends birthday is coming up and she told me she was inviting everyone from work. Which I was originally excited about because everyone was so close and I miss them all. Then she said "also just you know Emily is going to be there but you HAVE to be there! I'm gonna be so sad if you don't come!" I didn't say anything as I feel immature and it's not my birthday. She should feel free to spend it with whom ever she wants. So I told her I would be there.

I talked about it with my boyfriend and I told him I was uncomfortable having him come with me. He said "don't worry babe I'm not going to spend my time talking to her or anything" so I flat out told him he was not going with me as I am not ready for that with the place I am in with my healing. The more I thought about it... I can't go. I don't want to... I'm too hurt. I see that girls face and I feel filled with rage even though she did nothing wrong. Considering that I am considering telling Stacie I am going to need to sit her birthday party out. Then let her know that when she is free I'll take her out to dinner or something to celebrate.

I'm just worried this is hurtful and immature of me. Would I be the asshole if don't go to my best friends birthday party?