Soberwoilf is right - noones owed an essay on this.
'I can't go, sorry' is enough.
I used to feel very self con scious about turning down invites or not drinking...It was a while before I learned that noones owed a long story about me and my alcoholism.
Not only that, but most peoples eyes glazed over - they're just not that interested or invested in me and struggles....
D