How about
"no thanks, I've had enough"
"I'm saving room for dessert" (and then don't eat what you don't want)
or you could go with a real crowd pleaser with
"um, this doesn't taste right to me. Do you think it has spoiled?" LOL!!!

I'm sorry you're worrying about having others focus on what you're eating. I try not to anticipate what's ahead, and that helps some. Maybe just take it as it comes?
Hope you are able to enjoy the day.
partlycloudy