My girlfriend, who is 22, disregarded one of my boundaries. How can I work to repair our relationship?

This is a new experience for me, as I’ve never sought help from strangers online before. I've spoken to some friends and family about my situation, but I would also like to get perspectives from others I don’t know. Here's what’s been going on: My girlfriend and I have been together for nearly three years and moved into our first apartment about six months ago. Prior to that, we had a long-distance relationship, with her living just over an hour away. We kept in touch daily on Discord, spending hours playing games or just hanging out. I’ve come to love her deeply, and I feel that our feelings for each other are mutual. However, issues began to arise two or three months after we moved in together. She had been writing roleplays within the Naruto universe with a close friend, putting a lot of thought and time into it. After we settled in, she began looking for new partners online for more stories. I was fine with this since it was a hobby of hers, but I made it clear that I wasn’t comfortable with her including any sexual content in these roleplays, as I wouldn’t feel comfortable knowing she was creating intimate scenes with someone else. Just to note, my girlfriend tends to be shy about sexual topics with me. We don’t engage in sexting or send nudes because she doesn’t want to, and that’s why it would upset me if she shared intimacy— even through characters and stories— with a stranger online. She agreed to my condition, and we moved on from the discussion. After a while, she started to write with two regular partners, one man and one woman, and even spent some of her free time with them. At first, I didn’t mind, but things changed when she began to call the guy almost every night. I was okay with it until one night, she was in the bedroom on a call with just him, and I felt a bit uneasy. While going to the bathroom, I couldn’t help but eavesdrop a little. At first, their conversation was casual, discussing family matters, but then I overheard a strange tone from the guy. I only caught the word “clit,” and my heart sank. It got worse when my girlfriend responded with “fuck me.” I confronted her. She was fully dressed, just sitting on the bed, and claimed it was all for their story, but that was precisely the boundary I had set. We had a discussion about it, and I was willing to forgive her, but I told her I wasn’t comfortable with her maintaining contact with that guy. She assured me she understood and wouldn’t continue talking to him. However, my suspicions lingered. Just a few days later, I unlocked her phone, checked Snapchat, and found that the chat with him was still active, with her last message sent only an hour earlier. We talked it over again, and I forgave her once more. Now that I’m writing this all down, I can’t believe I didn’t end things after finding the chat, but my feelings for her are just so strong. I hope someone out there reads this lengthy account and shares their thoughts with me.