On the fence between her or my girlfriend?

In the final quarter of 2017 I met a friend of a friend. We went on a group date and then to a university society session which we were both part of. After that we went to the student bar and left early to get some wine from my place. I suggested we drank at hers cause it was less walking for her and it was winter. After we get to hers with the wine we sit downstairs for a short while. Growing cold due to no heating, we retreat upstairs to her room. There we play some videogames, she defeated me. As the night grew long I suggested I left. Reluctantly she agreed. The next day we met again as a group and had a meal in town. Sharing a meal due to funds and leaving early, we went back to hers and went straight upstairs. We played half the amount of videogames before I took the risk of kissing her. After some time of intimacy we both stood up, she told me she was not comfortable with going 100%, we agreed on going 50% if that. She performed a sexual favour to coin it as such. Towards the end of the night I was overcome by severe guilt following what had just happened. I revealed to her that I was in a 1.5 year long relationship. She was devastated, I said that if she didn't want to see me I would disappear. She said we can remain friends. After a few weeks she failed to attend the society sessions however her friend who had heard about it from the girl I disappointed (ref: A^), voiced her anger at me. Me and her friend were friends also. I felt guilt for both sides. A^ eventually returned to the sessions, I steered clear to show her I had accepted her warnings and disapproval. Fast forwarding to the present. Me and A^ have become more intimate again but nothing of calibre to the above mentioned night. As groups we all would go to clubs. 95% of the time me and A^ would travel home together. Me and A^ have fallen asleep with one another and spent hours together. This story has been condensed so much for the letter count. My girlfriend knows. I told her. Do I stay or go?
Updates:
5 mo
I have a week before A^ graduates. I've developed poor mental health due to this. I'm still with my girlfriend but am unhappy, I cannot find the courage to split.
5 mo
I know its unforgivable and I now hate myself, do I confront the situation or do I let it calm and grit my teeth and let nature run its course?
On the fence between her or my girlfriend?
1
0
Add Opinion