I think that you would be disrespecting your boyfriend, and yourself. Dating, liking, or even wanting to be with your BFs best friend is something a young naive kid do. Even if the feelings were mutual, how could two people that are very close to him do something like that. If you got together with this other guy; you would ruin a good friendship, and you and this guy might not stay together forever....where does that leave you??? They might reconcile, and you might be the one who's being talked about by every guy, and girl you know (trust me it happens). If you're so unhappy with you BF, let him go because he doesn't deserve someone who's willing to breakup with him for his best friend.
I hope everything works out.
