Boyfriends

So me and my boyfriend have been fighting a lot lately and i Decided to ask him if we should break up but we never finalize if we should break up or not so later that night he decided to get really fucked up and kiss a girl and sadly the same girl he kiss is the same one he fuck before we got together , and me and her been bestfriend since 5th grade , and she went on to him and then it lead on after that. What should i do? the message in white is my ex and the one in black is the girl