It kind of sounds like you have cold feet. I wouldnt say that its not normal to be feeling the way you do. You're young and saying that youre only going to be with one person the rest of your life is pretty scary. I would really encourage you to stay faithful to your fiancee. It wouldnt be fair to her if you cheated on her. If you're having second thoughts about spending the rest of your life with her you need to address those now rather than later.