being a puta (whore)

So we had a party at our house and my boyfriend passed out. His best friend ended up staying the night like he normally does. I was so horny and I couldn't help it I ended up giving him a bj and having sex. They're like brothers and I'm afraid he's gonna tell my bf. he keeps calling me and says he wants more and that I was the best he's ever had and that I have given him the best bj he's ever had. I regret everything it was just a drunken mistake. Should I confess or should I trust this will stay between the 2 of us. I really love my bf and want to marry him.