Oh man, I've found that people (not necessarily sugar) can be the biggest obstacle. Peer pressure is REAL. I find I can usually just say " no thank you, I'm feeling quite full" or simply a straight-up "no thank you". If people can't respect one's decision then I think the problem lies with them, to be honest. I don't like feeling I have to justify why I don't want cake/sugary stuff, so I try and just say "no thank you". I hope this helps!

Find the one thing you would either not enjoy, or enjoy the least and carry it around on a plate. Mash up or break off a piece of it so it looks like you are in the process of eating. If people see you already have food, they rarely push more on you.
I knew someone who would also do things like take the smallest piece of dessert possible and find a moment or place when no one was looking and then pour so much salt or pepper over it that it was inedible. He would either rub it in so you couldn't see it, or he would cover it with whipped cream to hide it. He could then walk around and socialize and not have to worry about the food or the people.

Festive drinks help too. Carry around a glass of whatever is being served or make your own mocktail. Get sparkling water (flavored or not) and toss some fruit or mint or anything you like in it and if anyone offers you dessert tell them you are going to have a piece "as soon as you finish" your drink - Keep filling your glass.

Most people don't care what you have (drinks or dessert) as long as you have something and are socializing with others who have the same