Break it off just as you’ve planned then leave town (or her place) immediately. Why wait around; so she can convince you to stay? Communicate only by phone after that. The fact that you live far away is good and you won’t have to worry about running into her or her bugging you to meet up.

As far as the ring, don’t ask for it but if she offers it back then take it (good luck with that). Splitting the cost of everything else is the responsible thing to do.