If somehow, in your stressed out state, you do forget something on your list, keep things in perspective. Allow yourself a good ten or fifteen minutes to feel completely enraged over the suckiness of your world at this moment. Then remember that in the course of your entire life, this moment will not make or break you.
If your boutonniere is locked inside the flower shop, call up a friend or neighbor who has a garden and ask them if you can use one of their flowers. Cut the stem at a diagonal and push a straight pin through the stem. If may not be perfect, but then again, nothing is.