You're weird. She asked you if you would get married in the future, you said yes, she asked you if its okay to address you as her "fiancée" to her friends, and you said sure. I dont particularly agree with her approach, she shouldve waited for you to propose. But you basically told her, yes we will get married, yes you can tell your friends we will get married, and now you're "Im not even sure I want to marry her". Bruh. If you arent sure, them you shouldve told her you need more time to decide and dont know if marriage is right for you. I also think, after 6 years if you arent sure then maybe this isnt the right relationship for you and its time to walk away.