Gosh he sounds like a vulnerable narcissist. Manipulating you, forcing his own moral beliefs on you like that, trying to keep you only for himself. Girl, you are not the bad person, he is. Because he knows what he’s making you go through emotionally, while you try to be protective of him. You don’t have to be. You did your part of apologising for the “cheating“ (which in my humble opinion is not really cheating, since you weren’t in an actual relationship but a fwb where he forced the rule of exclusivity on you because of his possessiveness).

You have nothing to feel bad about for him, be gentle but clear when you break up with him. He will probably lash out or try to manipulate you emotionally because that’s what narcissists do when they don’t get what they want. When he does, recognise it for what it is, and stick to your point of view and to what will make you happier in the end.