AITA for refusing to attend my best friend’s wedding after she stole my fiancé?

I (30F) have been friends with Mia (29F) for as long as I can remember. We’ve always been like sisters—best friends who’ve been through thick and thin together. From high school crushes to college heartbreaks, we’ve shared it all. We even promised each other that we would be there for each other no matter what, especially during big moments in our lives.

Mia has always been the life of the party, the one who could light up any room with her infectious personality. I never thought much of it when she would flirt with my boyfriend, Ryan (31M), especially since they were just friends. I trusted Ryan completely, and I trusted Mia too. We all hung out together, and I never saw anything that made me feel uncomfortable.

A year ago, Ryan proposed to me. It was everything I had dreamed of—an intimate proposal by the lake where we had our first date. I was over the moon, and so was Mia. She was my maid of honor, and she even helped me pick out my wedding dress. We were a team, or so I thought.

Over the past few months, I noticed some odd behavior from both of them. Mia seemed to be spending more time with Ryan, texting him frequently, and even calling him late at night. At first, I shrugged it off. But then, I started getting strange vibes. One night, I caught Mia and Ryan alone in the kitchen, laughing a bit too intimately. When I walked in, they both seemed nervous, and Ryan quickly changed the subject. I tried not to read too much into it, but my gut told me something was off.

A few weeks ago, I came home early from work to find Ryan and Mia in my apartment, clearly hiding something. I found out that Mia had been cheating with Ryan behind my back for months. I was devastated. My best friend and fiancé—both the people I trusted the most—betrayed me. Mia admitted to everything, but her excuse was that she had “fallen in love with him” and couldn’t help herself.

I broke off my engagement with Ryan, and I told Mia that I couldn’t be friends with her anymore. She begged me to forgive her, but I couldn’t bring myself to do it. What hurt the most was the betrayal from someone I had considered my sister.

The final straw came when Mia sent me an invitation to her wedding—yes, she and Ryan are getting married. She had the audacity to ask me to be part of her bridal party, like nothing had happened. I couldn’t believe it. She wanted me to stand by her side while she married the man I had loved and trusted. I couldn’t even think of forgiving her for the betrayal.

I told Mia that I would not attend the wedding, let alone be a part of it. She called me selfish and accused me of ruining our friendship. She said, “If you were a true friend, you’d let me be happy.” I’m struggling with guilt because I know she’s hurt, but at the same time, I can’t bring myself to be part of her life anymore, especially after what she did.

Now, my mutual friends are divided. Some say I’m justified and that she’s the one who ruined everything, while others think I should just let go of the past and attend the wedding to show that I’m the bigger person.

So, AITA for refusing to attend my best friend’s wedding after she stole my fiancé? Should I have let go of my pride for the sake of our friendship, or was I right to walk away completely?