Cheesecake, I'm sorry to hear about your anxiety in the run up to what everyone assumes is a wonderful, happy business.
I experienced similar feelings, waking up every morning for months beforehand with queasy dread. On the actual day, it turned out that the parents of the happy couple were NOT, in any way, the focus of attention! Which was good!
I do recommend Bach Rescue Remedy, available from chemists. The front of the bottle says "To comfort and reassure", which I find it does. I also decided beforehand that I would only have two alcoholic drinks during the day, as I couldn't bear the thought of being the worse-for-wear so publicly and in photos. I'm glad I did, as I felt very in control. No one commented if I chose a soft drink - and in the end I did enjoy a cheeky third one later in the evening!
I hope you'll feel reassured by the experiences of the other grans here who have 'gone before', and can wear a nice outfit, a big smile and know that This Will Pass.