Learn a lesson from this. Stop talking about "spending the rest of your life" with someone and just live your lives. You will find that the relationship will naturally move forward or it will end without you having had made promises and said things that end up not materializing.
I think she should have known you were not into marrying her the minute you "told her to go look at wedding rings" instead of just surprising her or.... going together to look at them.
Don't feel guilty though. If everyone stopped the run-away-train and got off before they married when they were having doubts, there would be far fewer divorces taking place.
Do her the kindness of going zero contact with her so that she can more quickly heal from her disappointment as well as the withdrawl from the habit of having you in her life.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion