And yet she still chose you for any number of reasons (you have memories together, you've made mistakes together, insert whatever sentimental nonsense here). I don't know. You also supposedly gave her another chance when you didn't walk out and yet now you feel the need to change your mind. I just find it petty--why even bother in the first place?
You want an easy way to get out, hopefully by not hurting her feelings--it doesn't work like that. If she loves you, it will hurt. If she chose you over someone else, it will hurt. If you're not prepared to make the relationship work TOGETHER (therapy or whatever the hell else it takes) before giving up, it will hurt.
There is only one way out and that is to say, "Sorry, I don't want to be with you any more."
Don't procrastinate and hope for a quick fix, there isn't one--you have made your decision now stop dragging it out like a coward.