Threaten to elope and watch her come around. lol
That being said, she is your mom so you have to be very tactful as she is emotionally and financially vested in these decisions. You will need to bend a little here and there, but on most choices, does she really have to have it her way? Wouldn't she rather you be happy, than she "be right" all the time? Is this how she normally is?
You need to talk to her about this before this goes on any further. Tell her you value her help and suggestions, and above all, "need" her to be involved (parents need to hear this) but you need to make the final choices because it is your day, and you too have pictured things a certain way. If she has a budget for you, tell her that you will adhere to it. You will also need to address the idea of "traditions" and talk about which ones you will or will not do. Like it or not, you will have to bend in some areas. As much as it is your day, she too has been dreaming about and picturing this day for as long as you have been alive. There are some things you should be sensitive, but pick and chose them.
It is complicated, not at all as easy as they make it out in the magazines and movies. People will get offended, just try and politely listen to people's ideas, and then do the right thing. Good luck!
