Hi,
I'm not married but as a Christian with three children by two fathers, neither of whom I was married to, I can empathise with that nagging guilt of not 'towing the official Christian line'.
I think the things to remember is that, as human beings, we are all going to make mistakes and we are all incapable of perfection. Life happens, sin happens and, if you believe in a compassionate and loving God, then that is okay. The whole point of Grace is that everything we do in life is covered by Jesus' sacrifice - all we have to do is accept that and keep on trying to deepen out relationship with him.
It is one thing to believe that marriage is a contract made in the sight of God to stay together for life but quite another to believe it to the extent that there are no circumstances that can justify ending it. What is the point in staying in a marriage that is completely loveless and causing misery on all sides? Where is the benefit in staying in a marriage where one partner is abusive or has cheated (even the church 'allows' divorce in the latter case).
I don't think it helps to see Christianity as having 'rules'. I believe the Bible sets out guidelines for what was the most likely way to be able to lead a happy and spiritually fulfilling life. I think some of it remains as relevant today as when it was written but that other parts cannot be separated from the culture and time in which they were written.
Too many Christians beat themselves up for 'breaking rules.' I think all that is needed is an acknowledgement to God that something in your life is not going the way he or you would want it to and to pray for his help and guidance in moving on and getting through.