I think Squeek has the right idea. When there's this much distress going on, it might be a good idea to let yourself be babied a bit, just curl up with stuffy and blankie and a soothing hot drink.
Do you understand what you need to do in the wedding? Were you able to participate in a rehearsal? Try not to worry about it too much. I'm sure you will be able to do your part. Even if you would make an error, it wouldn't ruin things. The wedding is for your best friend, right? I'm sure she would forgive you if erred. Don't be afraid to ask others in the wedding for reminders of what to do if you need to. Everyone participating will be nervous too, so it will be good for all of you to support each other.