Most Helpful Girl
Be kind, explain why you don't work out without putting all of the guilt on her. DO NOT break up via text or phone, only face to face is decent.
But the truth is that, no matter how you do it and what you say it is going to hurt her. You just have to remind her that the longer you drag it out, the more it is going to hurt and that you don't do anyone a favor if you're in a rs with someone you don't want to be with.
Good luck to you!