I like my boyfriend's bestfriend. What to do?

My boyfriend and I are in a relationship for the past 4 years. (We were in a relationship before that, he cheated several times, we broke up and this is actually the second phase)
He has a bestfriend from his childhood days and I've actually met him before and exchanged a few words but that was it. I never liked him in anyway, far away from liking him romantically. He talks less and is a bit inconsiderate.
But recently there has been some cases of my boyfriend lying to me and stuff and simultaneously we have spending more time with this bestfriend of his as I hang around with his group. Recently I have noticed that he can talk to me more freely when he is drunk and doesn't talk much otherwise. He is also actually really caring but his private nature gives an impression otherwise.
I have been having feelings for this guy for quite a time now. I often fantasize him when I am alone and recently I kept thinking of him when my boyfriend and I were having sex.
Although I love my boyfriend very much, I am actually often troubled by thoughts of him cheating on me most of the time which is actually affecting our relationship. I know he loves me too but he gets too flown away at times really easily and regrets and pays for the consequences after.
I don't think his bestfriend actually thinks of me in the same way but he gets friendly when he is drunk and takes care of me.
I am not sure what to do with my feelings. Are this only infatuation and are these feelings only for physical attraction? Or should I actually do something about it?
Not to mention, i have never had real boyfriends before this guy. I mean I fell for one when I was underage but never had a physical relation(not even a hug) , and there was another guy I just made out with once. That's all