I think your answer is right here. You are afraid that he will leave you. As you get closer to marriage, the last big step... you are getting closer to risking even more trust that he won't leave than you need right now in the fiance stage.

I mean really, how much do you have to trust a guy to never leave you to go on a first date with him? None, right? How about going steady, if they still do that anywhere. You're going to need a little trust now, it'll be a little embarressing if he leaves you after he asked you to go steady - you thought it was serious. Then he asks you to marry him - major trust now...but actually saying "I do" requires the most trust.

You're getting closer to the relationship final exam and it's natural to have some stress even if it's your favorite class.