I cheated on my fiancé

How do I tell him? I cheated because  I wanted to, it was with an ex that I've wanted to be with. I never stopped thinking about him, the reason why we broke up was because I was too young. Now that I'm 19, my ex wants to be with me and says he never stopped loving me. I can't keep lying to myself. I need help, how do I tell my fiancé.? We also have a 6 month old son together to make it even harder :(