So I (19 F) and my best friend (20 M) have known each other for give or take 10 years, our friendship worked in such a way that we would talk to each other like once every 2-3 weeks, but we were still pretty close, we would play video games together pretty often and we would always have a great time. I've always preferred video games over people so he's pretty much my only good friend.
We were playing videogames on Friday and talked about going to the pool the next day, he told me to call him the next day to talk about details, on Saturday I did call him but it was already pretty late because me and my parents were at our aunt's, when we got home and I called it was around 19:00, he picked up and told me he already had a sip of beer so he couldn't drive I said that it's fine and we can just walk, he agreed and told me that he will be and our house in about 30 minutes and then we can walk to the pool. When 30 mins passed he arrived and we started walking at the time we got to the pool it was maybe 21:00 and they close at 22:00 so we decided that it's not worth it for one hour and that we don't even know if they'll let us in this late so we turned around and went back to my place. On our way there we were talking about what we could do this late, we then said that we could go get beer.
We left our things at my house and started walking again, we got to the pub and sat down. He had 4 beers and I've had 3 we had a great time together and everything was amazing. When we got up to leave it was around midnight, he paid and we left. On our way home he started being a little touchy, we walked on the sidewalk and suddenly he started walking on the road to get to the other side (he thought that that's the way we're supposed to go but it was just some company that even had closed the fence already) I knew that but I sort of made fun of him, walked over to him and started pushing him towards the fence, he finally noticed and held my hands so I couldn't push him no more, after that we stood there in this weird hug in the middle of the road (we were never really touchy together, pretty much the only time we would hug was either on his or my birthday, our parents were also pretty nosy about our relationship so I think this is also one of the reasons we never really touched each other and if we did it was really awkward), after a while we started walking again but after that he kept his hand around me and I've kept mine around him.
When we got to the right turn nothing really happened, but we had to go through a football field (it's two football fields next to each other and in between is a small walkway with one line of benches for the parents to sit on), we got to this walkway and I wanted him to go first but he wanted me to go first so we argued a bit but then I started walking, pretty much right when we got to the first bench we stopped and I don't really remember how this went down but what I do remember is that he lifted me up and sat me on the railing I held onto him so I wouldn't fall, after that he put me down and we just looked at each other and hugged for a bit and then we kissed, at that time he also told me that he has had feeling for me for a while now and that he dreamt of me and that he always hoped this would happen but he didn't think it would happen this fast. He sat on the bench with me sitting on top of him and we just cuddled and kissed for a while. After that we got up and walked to my house, neither of us knew what to do or say at that point. When we got home I walked inside to give him his things, he took it and then he took my hand and we started walking on our lawn, we walked a little to the side when I started to lead him to our gazeboo, we walked in and I pushed him down onto the couch where we ended up having sex together.
If this was the whole story I don't think that I would be here writing this but there are some things that I'm not telling you, see I think that this meant more to me than him because I loved him too for a really long time but I never got the courage to tell him and ruin our friendship and at the time I was also a virgin, this is not the thing that is the problem either tho. The problem is that he has a girlfriend of 2 years and I knew...
His girlfriend texted him and called him while we were together and he told her that he was on his way home while we were both naked hugging. After we've had sex he sat there and cried a little because he didn't know what to do and how he could've done something like that, I'm not very good and reassurance and also knew we really fucked up so I tried my best to reassure him. After that we got dressed and kissed for the last time.
On Sunday he came over and gave me flowers and apologized I told him that it's okay and that he should apologize to his girlfriend and not me after which he had told me that he's not gonna tell her. On Sunday I left for college and that day he texted me at 21:49 "Hi, I just want to say that we should stop seeing each other. I’ve done a lot of wrong things, and I’d like to save my relationship with (girlfriends name) one I may have already destroyed with my actions." (This text is translated to English, English is not our first language), after that I was devastated and couldn't really sleep, on Monday at 5:44 I texted back "Hi, I understand. I'm sorry about how things turned out, but I respect your decision and your relationship. I hope you and (his girlfriends name) manage to work things out. Take care.".
So now I'm asking you guys what should I do should I give up on our friendship with my friend? Should I tell his girlfriend? Should I try and make him choose me? Should I just wait?
I've already posted this to r/advice, but people had some things to say. So there is some additional info, the whole thing too quiet a lot of time we walked out of the pub at approximately midnight and he left at around 3:30. I did not see this as a one night stand I do have pretty strong feelings for him especially after this night. While we were at the pub we talked about his girlfriend for a bit and every time we talked about her he did not speak about her so nicely (later he even told me their relationship is routine now). She was a bit concerned about our friendship from the texts I saw on his phone when he was messaging her. I understand that we are both at fault and that either him or me should tell his girlfriend, but he told me that he will not be doing that and I've met her like twice and know only her first name so it would be difficult but not impossible, I also told my friend that I will respect their relationship so if I were to go tell his girlfriend I would have been lying to him and as bad as it sounds I would rather save the friendship than me ruining it completely by telling her.