I have been in that situation before. In the end the only thing I could do was be completely honest and tell the person that I just didn't want to be their friend anymore, because x, y and z. You know what? The lead up to it was wrose then the actual 'disscussion'. Once I had told her that, I felt liberated and she took it quite well, I think because I had been honest, rather then just becoming resentful and nasty and petty.

I would suggest ringing her up to talk to her. If you need you could write up a script or list of the issues you have so that you don't get flustered (and she won't know you have a list if you are on the phone). Be prepared that she may try and talk her way out of it and minimise things. I always fell back on "You are probably right (about whatever), but I cann't help my feelings whether they are rational or not, and I would just like to end this friendship now while I still have good memories and haven't become too bitter and negative."