Have some self respect and leave. Don’t use his continuous cheating as an excuse to make your way in as well. Let this issue stay between he and his wife and stop being part of the problem. Leave without telling him why, and leave that family alone. This is coming from someone your age as well.

Issue 2: you feel stuck because you feel attached and perhaps there is some more going on with you that you lead on. No one wants to be with a cheater, and once a cheater... you know the line. So respect yourself, heal with a therapist or meet someone else, and see your friends every single day so you don’t feel lonely. Keeping distracted will help you leave this thing.