How to you break up with your BFF?

My best friend for 14 years is turning into someone I can't stand she is so cold and rude to people. I guess I never seen it before or I just looked passed it.  I've been not hanging out with her or I just don't answer when she calls. This last weekend I was out at a bar and she was there and started freaking out crying how I'm ignoring her. Truth is I am I just don't like to be around a rude person, I've told her and she just won't get it she thinks since we been friends forever that I'll always be her friend. I don't want to hurt her I feel like we just grew apart and arnt into the same things. It's so weird I feel like I'm breaking up with a boyfriend or something! Help what should I do?