I think you wanted this relationship to be something it just wasn't.
Sure, there were moments you felt special. But that is not enough when the overall tone is one of feeling unheard and unseen, and when you feel your partner doesn't respect you.
You didn't cause him to cheat. He chose that. If he was unhappy, he could have ended it with you before he had sex with someone else. He didn't do that. He is not a quality guy, and therefore you haven't lost much at all.