My concern is that you seem to know so little of the Bible that you had to come here and ask us what the Bible says about divorce.
No, it is not a sin to stay with an unbelieving husband. One of my best friends married a man before she was saved, over 20 years ago, and he is still not saved. But he loves her and treats her with respect, love and dignity. They have a wonderful marriage.
So that is not the question. The question is why are you staying in what is basically an abusive relationship, and why you have confused "love" with codependency? Or unrealistic expectations, in that you knew what he was about, and you thought he would change.
People do not change themselves. Only God can change people. But if that person is fighting against God, won't give up his sin, then he cannot and will not change.
You have to decide if you want to grow old with an abusive adulterer. Because YOU are the one who has the choice. It doesn't matter if he is the father of your children, it is probably better for them to be away from him, and the selfish behaviour he is exhibiting. But that might be water under the bridge for you.
You need to get see a counselor and figure out what you want to do. As everyone says, your husband has given you ample Biblical grounds for you to divorce him. Try and get someone who will help you see the reasons you stay in this kind of negative and abusive marriage. Then decide if you have the strength to hire a lawyer and end the nightmare it seems like you are in.
Oh, yes! And do get out your Bible and start reading it every single day - for the rest of your life! God is not just there to grant your prayers and then you go do your own thing - including disobeying him. Get to know God in his Word, and also in prayer. I'll bet you haven't been doing a lot of that, because it is hard to pray when life is a confusing mess.
And I will pray that God will give you wisdom and show you the way you should walk, whatever way that is.