I just discovered that my [m30] boyfriend has been unfaithful to me [f26].

I'm feeling really lost and unsure of what to do, as the title suggests. My partner and I have been together for about six years, and our relationship has been wonderful—we were even planning to get engaged soon. For a long time, we were in a long-distance relationship, only able to see each other twice a year. Thankfully, we closed the distance and have been living together for about a year now, which has been fantastic. However, today I stumbled upon a text on his phone from a girl I didn't recognize, and it instantly filled me with a sense of dread. The message itself seemed innocent, but my gut told me something was off, so I confronted him. Eventually, he admitted to having a one-night stand with this girl over a year ago, before we closed the distance. Apparently, she reached out to him again while in town, but he claimed he turned her down. He took full responsibility, explaining that they met at a bar during his vacation, and things got out of hand when he was very drunk. He expressed deep guilt afterward and hadn’t contacted her until her recent message. He insists he takes full responsibility and completely respects whatever decision I make moving forward, but I’m feeling so conflicted. I used to think that girls who forgive their partners for cheating were making a mistake, but now I’m not so sure. He has promised to do everything he can to regain my trust if I can find it in my heart to forgive him, and he still wants to marry me. Would it be foolish to forgive him? I can’t help but think that if I break up with him over what may have been a one-time mistake during our long-distance phase, I could be throwing away a future with a great guy. Yet, I worry that if I take him back, not only might he cheat again, but I may never regain the trust we once had. I truly love him, and I believe he loves me too, but this is a significant breach of trust. Has anyone experienced something similar? I know I’ll likely get a lot of “just break up with him” comments, and I understand that perspective, as I would have felt the same way before being in this situation. However, I’m genuinely at a loss right now.