I've done two big wedding cakes and I went through the anxiety with both of them. About a week or 2 before the wedding I start wondering why I ever said I'd do it. And who do I think I am that I can make something so important as a wedding cake!
But like others have said, just plan every detail out. Make lists, do everything you possibly can ahead of time. If you're concerned about a specific detail of the cake, have a plan B in place. Once it's over and everything works out, it's all worth it in the end.
I really think if you plan well, do your research to make sure the structure is sound and you support everything properly, you'll have success. Good luck and try to relax a bit and enjoy it