If I were in this situation I would end it. I would end it quickly, I would meet in person... be direct and straightforward that I was very sorry, and I didn't want to hurt her and that I felt things were going in a bad directly. I would then break off all contact (for at least 3 months).
Breaking up is a part of life. It sucks, but it happens all the time. The best way to do it is to do it completely with no games and no uncertainty. It hurts, but we all get over it.
As I see it, the only other possible solution would be call off the marriage and say you want to date for a while. I would only do this if I was serious; meaning I had real feelings for her and was willing to work to build the relationship.
Either way you have to be honest and direct. Anything else is dishonest and cruel.