How do I break it to him, what would you say?

I am physically repulsed by my boyfriend, I can't help it though... I used to think he was so cute now I'm trying to avoid him which is so stupid and childish, part of the reason though is probably something to do with the fact that I have liked my best friend Kieran for sometime now, I just don't know what to say to my current boyfriend, it's been a serious relationship for about 6 months now and I know that he's totally in love with me while I feel as though he repulses me and. Like my bestfriend! I don't know what to say when I break up with him, I've never been good at breakups and the worst thing is I'm bestfriends with his sister, but we hav always said no matter what happens well still be bestfriends,,