You don't need to address her formally 

Guys address their girlfriends very formally.They become very gentle and humble when they are with their girlfriends.But this is not the way they actually are in most of the cases.Everyone is informal with their friends especially best friends.This is the best part of being in a relationship with your best friend.If you are in a relationship with your best friend then,you don't need to address her in a formal manner.You are free to call her whatever you want to call her and she will not feel bad about that.