I believe being nervous before a wedding is quite normal and natural.
Just remember that, although marriage is a big decision, it isn't some decision that could make or break you I don't think. If it doesn't work out (unlikely) then not toooooooooooo much harm done, but it probably will work out and everything will be fine. All you're doing, when you get to the nitty gritty of it, is adding a new label on your relationship and elevating your relationship in legal status. Yes, marriage is more than that with love and stuff, but fundamentally that's what it is, so perhaps just realizing that might make it better? But I dunno xD