He had absolutely no idea that you liked him in that way, even if he had done, who says he still wouldn't have got with that girl? If you really like someone, you have to go for it because the longer you leave it, the more likely it is that they will find someone else. Let that be a lesson to you.
Don't try and split them up. Honestly, do you think you stand any chance of being with him if you behave in such a way? Of course not. He will never want you as a friend, never mind a girlfriend. Just because you don't like something, it doesn't mean you have the right to go in there and ruin it for other people. You are meant to like this guy, be his best friend. Best friends don't behave like that so don't even go there.
It is a difficult situation to be in, of course it is and there is no easy way of coming to terms with it but he has a new girlfriend now, that's not really any of your business. You have two choices, you either stick around and be his friend or be honest, and risk causing trouble. Being his friend is exactly what he would want, you two could still remain close despite him having a girlfriend but at the same time, getting things off your chest will be good. At the end of the day, it's up to you but please don't try and split them up. Imagine how you would feel if your so called friend tried to do that to you. I suggest you try and keep your friendship going, but it's up to you what you do.