I went through almost this exact same situation with my best friend, right down to the history of him cheating on his girlfriends. I told 2-3 others before I finally got up the balls to come out to him, and even then it was so hard that I balled my eyes out in the process. But here's the deal: If he's really your best friend, and he cares about you, he'll still be your best friend afterwards. Nine years later, he's still my best friend. And the fact is, after I told him, it made accepting myself a shitton easier.
As far as the attraction, here's how I made it through:
1) Firstly, whether he does so consciously or not, coming out to him will increase his awareness of ways he may have lead you on before he knew you were gay. If your friendship is important to him, he'll mitigate these without cutting you off.
2) My best friend is a straight man who loves me, and is atypically open with his emotions. He often tells me that he doesn't know where he'd be without me, that he trusts me more than anyone else (including his longterm girlfriends), that I'm one of the few stabilizing agents in his life. These are things that normal straight men say to the girls they love, NOT a best friend. Learn to identify and accept evidence of your tight platonic friendship as exactly that, and NOT a potential romantic doorway.
3) Assuming it truly is love and not lust, it's still okay for you to have feelings for him as long as you accept that they will never come to fruition. He is not gay, and can never feel that way about you. But it's not a crime to love someone, and as long as it doesn't keep you from living your life, can make a strong bond even stronger.
There are a lot more other things you can do, but none of them are actions (i.e. if you do this, it will all be better). Your mind and your heart have to internalize and come to terms with who you are. That being said, my best friend is still my best friend, and I still have residual romantic feelings for him. I've also dated other guys who I've thought were better matches for me. One of them I could even picture spending the rest of my life with. Before that guy broke up with me, my best friend confessed that he was worried he'd been replaced! haha