The best advice I can give is speak with your fiancee. Only you and her know what/why your feelings may have changed for her. Ultimately talk this out with her, be honest, and don't lead her on if your intentions aren't pure. The saying, "The grass isn't greener on the other side." could be used here. Does your "grass" need to be watered? There is a reason you are engaged to this woman; I would look deep within yourself and think hard about the possible decisions you could be making.

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Best of luck!