Oh Suzie, I am so sorry you are going through this.
He is just trying to force you to take the blame for his moral defects of character so he doesn't have to look at himself and take responsibility for his actions.
Don't let him dump on you like that.
Sounds like you are a great person, don't change a thing, stay who you are, don't change into what you think he wants you to be. He will still cheat, it won't solve anything.
Until he is willing to take a look at himself, he will continue to try and FORCE you to take the blame for what he is doing. Don't let him do that to you. He can only give it to you if you are willing to take it.