I'll break this complicated situation down down best I can.
So me and my long term gf have been undergoing hard times for a while, and about a month before I started uni we decided to have a break from each other, and she went home to her native Denmark. During this time, I was ready to leave her, and felt that a good week of partying and meeting new people would be a natural way of getting over her, as we hadn't officially broken up yet.
During freshers week, I met a cool girl on my course and on a night out with friends one thing led to another and I ended up in her room. I knew it would be wrong to do anything without officially ending it with my sort of gf but I was really up for it and was letting myself go. We got on really well, had a great night and I ended up staying there for another 2 nights.
On the 3rd day, she peeped on my facebook and noticed the pictures of me and my gf which were from fairly recently and I hadn't removed yet. I came clean and said the truth...that we are on a break from each other and I am ready to end it with her, and that I didn't tell her about my gf earlier as I really liked her and didn't want to ruin it.
Well, as it turns out, me and my gf had a long long talk and after lots of figuring out our problems are back on and she is now coming back to England to be with me. I regret the whole fling I had but it was on a break between us so don't think it was awful. However, the girl that I had the fling with hates me for it and is making out that I am a horrible person, and that I should tell my gf about what happened, and I feel that if I don't then she is going to expose it herself...which would be an absolute disaster.
The way I see it, it was freshers week... A lot of crazy things happen and i'm guessing a lot of cheating does too, and in a worse context than mine. I'm not saying it wasn't wrong and that I shouldn't have done it but it happened.
What do I do? I want to just put that to the past and work on making my relationship better but this girl won't drop it.