Most Helpful Girl
I'd go somewhere private where she won't feel embarrassed or cause a scene for starters. Then I would say something like "(name), I think we should break up. I really care about you but I just don't feel like I did when we first started going out..." Then explain really nicely and rationally and don't start yelling back if she yells at you. Also don't ask to be friends unless she wants to be or she'll probably get pissed because that's the typical break up. At least in my experience, this usually works.