Slept with someone with a girlfriend. What should I do?

So this was months ago now but I still feel super bad about it but please hear me out.

We we started talking and we met on a night out and I didn't realise he had a girlfriend but his friend did keep talking about a German girl. I went back to his and he kissed me, he made the first move. I went home and nothing else happened. We spoke some more and I thought this guy genuinely liked me. Then I met him after a night out when I was drunk and he was sober. Yet again he made a love on me and more things happen. He mentioned a girlfriend but said it was an open relationship. I knew the next day that I shouldn't have went anywhere near him if there was another girl in the picture. He didn't speak much after the second time we met and it upset me. I did really like this guy. So a few weeks later I saw him again and went back to his. I didn't realy want to sleep with him when he tried to have sex with me and I said 'we can't have sex' to which he replied 'get a taxi home the' so I was stupid and we did and we both said it can't happen again. I always wanted to meet him so I feel pretty bad about it. I know it wasn't just his fault but he made a move on me when he could have told me to go home. I am not saying I'm innocent. But now I keep thinking about this other girl as he still messages me asking to meet but I say no. I asked if he was in a serious relationship and if he had cheated on someone with me but he said not to worry about it and that he can't label their relationship and that I haven't done anything wrong.

I don't like him anymore, but I feel guilty for this other girl. What should I do? He did tell me it was open and every time I was drunk and he wasn't.