I feel that way sometimes too. I chalk it up to jitters. We are from very different backgrounds and each of us had a different idea of what out adult lives would be going into this. I truly love my Fi for all that he is and all that he is not. I tend to get too caught up in propriety and what other people think, whereas he could care less. He really does keep me grounded.
As much as he drives me crazy, I know he will make a good husband and father.
From what everyone else has told me, this is COMPLETELY NORMAL. You are about to make a huge step in your life and it is natural to question it. It would be the same if you were to take a new job, buy a new house, etc. There is a reason you wanted to marry this man, so remember that and take comfort in it.
Once the wedding is over you will both be ready to move forward!