Here's the deal – it's quite complicated.
My girlfriend (25F) and I (24M) were together for two years during the last years of high school, after which we broke up. An important aspect of the break up was long distance and trust issues concerning a guy she was there with. I must admit, I was young and extremely jealous, which I am completely over now. She ended up seeing this guy for a year nearly as soon as we broke up (over the phone…) and I was devastated by it. She never 'cheated' on me, but the spark probably started while we were together. I have forgiven her for this in the meantime.
Two months after we broke up and – as she was already with this new guy -, I had sex once with one of our mutual friends from high school. We were very drunk and it was a obvious mistake. I remember it as feeling very much hurt by the whole situation of the breakup. Not trying to say that I did it for revenge, but I did not feel any loyalty towards my (then ex-)girlfriend. Me and and the friend agreed to never tell her and were never intimate again. This happened 7.5 years ago.
Meanwhile, my girlfriend and I rekindled towards the end of our college years and are in a new relationship for two years now. We are very happy together and I (and she as well) see a long future together.
Lately I have been very torn about not telling her that I hooked up with her friend. Important to note is that my girlfriend did actually tell me when we started hooking up again that she had sex with someone we both know during our time apart. However, we both know that I don't consider this dude a real friend and I am not in touch with him anymore. And obviously, the loyalty issue with the friend I had sex with is important – if I tell my girlfriend, she will break her friendship with this girl. Also, I have gained knowledge that the girl did in fact tell people (I did not). One of my friends asked me about it, as his girlfriend told him that the girl had told her (they were college roommates). Everyone involved has strict instructions not to tell anyone, but from this they do not seem to take that too strictly… To make it worse, the girl and I interned at the same firm while I was together with my current girlfriend. We are friendly towards each other and never discuss what happened 7.5 years ago.
I am extremely scared that my girlfriend will find out at some point in our lives. I have so many regrets that I didn't just tell her when we started hooking up again. It would have been so much easier. 2 years later I feel like she's gonna feel very much betrayed. I do not want to give our relationship up.
What do I do? I have the feeling that at some point I must tell her, because if we stay together forever it will tear me up too much. Besides that, she might actually find out. What is a good moment? Maybe before we move in together – framing it as a next step in life and an important moment for me to tell her? Do other people have experience with a similar situation?
TLDR: I didn't tell my girlfriend I hooked up with one of her friends while we were apart. Now I feel like I should.
EDIT: Maybe it's good to add that my girlfriend is an emotional person. I fear for her reaction, as I can imagine she won't take it rationally and she won't be able to stomach it. What are your thoughts on my chances of coming past it with her, or will she break up with me?