First, we need to agree that there are no two relationships that are similar. Whatever you might be finding super disturbing in your relationship could be a nonissue in someone else’s relationship. Case in point, your girl having a guy best friend.
We all have that caveman level of jealousy but then jealousy is never attractive. How do you let your girlfriend keep this guy so close, probably a straight man who, no doubt, knows more about her than you do?