I kissed my best friends gf

I(16m) usually go out most weekends with some friends for a drink and they will usually bring their girlfriends along with them. My best friend had his backed turned to me looking at something next to my other friend. I was sat down with his girlfriend. We have always been friendly but that was it, just friendly. Out of nowhere she has her legs on mine and we just started kissing. It was going to go even further but we knew straight away what we did was wrong. I told my best friend what had happened and he took it surprisingly well telling me that everything was fine when I knew that it wasn't. I havent been this miserable ever and even though he forgives me I know I've completely messed up and I've betrayed his trust. We were insanely drunk(not an excuse)and we both have said it was a mistake and we don't see each other that way.