It's great two other have replied.
I'd like to point out that your standards for a monogamous relationship is a given, that it is expected in your mind and that is entirely normal. Your standards is what counts here for you, no one else.
We get this situation often. Some partners can live with their partner cheating on them other cannot.
You have left him on a previous occasion. I suggest that had you involved yourself with activities that distracted you, sports, hobbies, interests even dating, that you would find it more tolerable. I know what its like to be idle and think, think, think...then you miss the other facets of the relationship.
You are deserving of a trusting partner.
I hope I've helped. Repost anytime