But I love gatherings. Isn’t it ironic?
Sound like “goer’s remorse” to me. 
Or “ayer’s remorse” 
Just say nay.
This happens to me a lot. Though I love to be places, I hate the process of traveling. So the event sounds good; but getting there and back causes the pain.
How is clitorall poettry formed? How girl get pregret?
Nostalgia is the pain of returning home. So I propose enodalgia, the pain of visiting.
Not for the relief you feel, but the word for what turns one of these nightmares into actually quite good fun is “canceled”. 
That’s me. When I accept the invitation I’m thinking of the good parts but when it comes to the night before the sheer faff of it all overwhelms me.
Cold feet?
My new word: Irfday. Any day of the week that’s not Friday. Rhymes with “Earth Day”.
Yesterday, Friday, I started off the day thinking it was Saturday. Over and over when working out what to do when I thought it was Saturday. E.g., “If I wait until tomorrow (Sunday) to shop there’ll be fewer people in the stores.”
This went on until sometime mid-afternoon when my efforts to persuade myself it’s not Saturday paid off.
Except it was now Thursday in my mind. Grrr.
It appears that my mind was going to think it was any day but Friday. And the ideal term for such a day dawned on me.