My fiance wants to break off the relationship, because of something that she thinks iv done?

so i send a text messege to a friend of mine thats a girl, asking hows shes doin, replayed good and you, my response was livin life... now my fiancee thinks that im tryin to cheat of her because i didnt tell this friend of mine that im ingaged.... we had a long fight last night and now i feel that she wants to end it tonight.......what should i say or do to make it better?