"Paul says in Galatians 3 all Law ended with the cross."
No, what it says is that the law was to teach us, but when you get out of school you don't need a teacher anymore. You keep the law without having to have it spelled out for you.
Gal. 3:21 Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.
In other words, if Adam hadn't sinned we would have been righteous by the law. But because there was sin only Jesus could restore us to that point. There is no law that we could keep that can take away sin. Law can only prevent sin.