Bluegem:
Lot's of helpful hints here.
I don't know if you have ever practiced visualization, but if this type of thing works for you, try visualizing yourself at the reception and how you will handle things. If you still feel uncomfortable on the morning of the wedding, listen to that voice and don't go. If you feel strong that you can handle things, celebrate your friend's special day.
How are you going to handle the toast to the bride and groom? Figure this out beforehand so that you are not caught off guard. If cravings come, go to the ladies room, out on the dance floor, listen to your therapist and know that the cravings only last a few minutes.
Know that you can leave at any time and most importantly, do it if that's what it takes not to have the first drink, then the next and the next and the next.