Your fiancé/partner blacks out and kisses someone else, so you

My fiancé NEVER does this.. he took it too far and got too f**ked up and doesn’t remember anything at all. He is a recovered addict and relapsed, wrong place wrong time wrong people. I was working late and he’s been struggling emotionally/mentally. He kissed this alcoholic girl who also does drugs most days, who’s his friends roommate and he doesn’t remember anything. Only reason we figured it out is because they had texts between the two that night. He got home and couldn’t even stand, it was really heartbreaking.. he was sobbing the next morning saying he didn’t mean to and had no idea and it’s completely out of character. Initially I was going to leave right away, but it’s easier said than done. We’re in our 20s, engaged and live together. We have fur babies, and he’s been my bestfriend since we were 12. We’ve also been ttc for 3 years. I am so lost on what to do... I don’t deserve this