"By now you should know me better than that" meaning that the girl should know better than that. That he is willing to go back to the girl. "But no one can stop me now, it's like that" meaning that he is going to make his own decisions. There is no one that can stop him from doing what he wants to do. "When I walk away I will never turn back" meaning that once he leaves he will never come back. She may think that he is going to come back but he is not going to.