his friend.

So my fiancé has a female (best) friend that he used to want to have a date and marry before me. I mean she helped him pick out my ring and I know they aren't doing anything so most of the time I'm cool with them hanging out but sometimes he can mention her name or say he's going to have lunch with her and it's like he's taking my heart out of my chest and stepping on it. What do I do? Has anyone else been in this place?