I think a lot of people have that problem, especially if you buy your dress well in advance of the wedding. Sometimes the more you look at something, the more unsure you become. I think that goes for a lot of things in life.
I know that when I first tried on my dress, I fell in love with it. Now, I see other dresses that I like and wonder if I made a big mistake, but I then have to make myself remember how sure I was when I had my dress on.
I'm sure you'll be absolutely beautiful in whatever dress you choose!