How can I avoid ruining my friends wedding?

My mate is getting married soon and I've been invited, along with the rest our group to the full day. Unfortunately, I have a habit of ruining things whether through social faux pas or just through my presence. I usually come up with an excuse for important social events which eliminates the risk of me ruining them and just go to informal things like pub visits, avoiding stuff like ski holidays. However, when I was asked outright if I wanted to go I was on the spot and couldn't think of an exucse so said yes. How can I avoid ruining this important day for them?