I have never had feelings like that for a best friend - if am happy with someone i don't expect people especially friends to intrude or sabotage a good/loving relationship - it's crass,offensive,selfish and senselesss

Take it from a guy who's been through something just like this: Do not get involved until he decides to leave the other person of his on volition. You would be amazed how easily your most trusted friend can turn into your most persecuting enemy on so slight a provocation. Think of him: he is happy, so leave it be. Though it may not seem like it right now, there are plenty of guys just like him. I had my brain and heart ripped in pieces; for everyone's sake just let him be and Make it no big deal.

He is dating someone else. Despite your feelings, he is being honorable to the woman he has chosen.

YOU NEED TO BACK OFF. He's taken. If you can live with BFF, keep it, but it sounds like you want the whole enchilada or nothing.

Move on and find someone who's actually interested in you.