In one of my psych textbooks, it says that it's better to dig down and truly FEEL your emotions, and not to let them be repressed. I'm pretty sure that's what guys tend to do naturally, but don't do it. If you do, then it will come back to haunt you, or it will get released in a destructive manner. Like if you flick a can of pop that had been shaken up too much.
Let's face it. She was your best friend, so you already had a really strong report with her even before you started dating. Maybe you feel betrayed by her, and you're angry. If you are, but don't want to accept that, then those feelings get transformed into sadness and confusion.
But while you're digging through painful memories, it is important not to get caught up in self-pity: instead, discover WHY you feel the way that you do, and then come to terms with the fact that you have a particular emotional response. When you have a source, then you can discover a solution to it, because you're taking control of how you feel instead of letting it control you. And you can't do this just by saying to yourself "I choose not to have feelings for her anymore." Personal experience: I fell for my best friend too, but refused to accept it. Things then just kept getting worse and worse until I feared that my hidden emotions would tear us apart, but as soon as I realized that yes, I was in love with, and felt very protective of, my friend, I could get over it. Doesn't mean I still don't have some vestigial feelings, but it's not so bad that we can't spend time together.
Just a bit of psychoanalysis for you... could you hate this new boyfriend because you feel like you could be better for her than him? If you do and you recognize it, then perhaps you can have positive interactions with them because you know that how you feel about him is coming from within and not from him.