She offers some good suggestions, but I have some additional ones.
Set up a safe call. Arrange with a friend to call you at specified intervals to make sure you're doing OK. If you're not, the call can serve as an excuse to leave. "Sorry, I've got to leave. That was work/friend/the FBI with an emergency, and I'm needed."
Wear a hat. The brim can help hide your expression, which gives a kind of psychological distance. It also increases your apparent size. By the same principle, wear clothes that make you feel strong, competent, and adult.
Make a family-craziness bingo card. You know how you can predict that certain events always occur at family gatherings? Aunt Alice and Uncle Bob always get into a fight, Cousin Charlie makes nasty comments on your job, and Grandma Dorothy fusses because her daughter-in-law keeps interfering in the kitchen and nobody's helping her. Make each of these events a square on your bingo card. This can defuse some of the tension of waiting for nastiness to happen. It also makes you more an observer, not a participant.
What are your ways to handle those family gatherings?