Family celebrations are a tough one!
I often excuse myself to the bathroom, ground, breathe, and then go back. It helps me get through to have even just 2 minutes to myself, even if everyone now thinks I have irritable bowel syndrome because I'm sneaking off to the bathroom so much. If you can take short breaks, this may help you get through.
Another thing that helps, is for me to play the goofy photographer. I go around taking pictures at family events. Somehow, this helps me slow down, and it also distances me from people a little. If kids are around, I often spend a lot of time being around the little ones. I will push them in the swing or bring printouts of pages online for them to color. I usually end up coloring with them, not in a regressed way... But they think I'm the best aunt and cousin EVER, and the adults are not as obnoxious to me when I'm with the little ones.
I also carry a stone in my pocket. I have a rock from a place that is safe and peaceful, but there have been a few times I have picked up any old stone. If I don't have pockets, I keep it in a purse. It is a tangible reminder to of the world away from my family. It's also soothing to hold it in my pocket, and no one can notice.
Another thing that I do is play mindfulness games in my head. Like I will count or notice everything that is green in the room as my aunt rants on, once again. Then I will notice what is blue. Or I will try to notice the way my sweater feels so cozy and warm as my cousin acts like jerk and I feel overwhelmed by the sheer number of relatives. Mindfulness can help regulate anxiety and other feelings, and it's about noticing, as objectively as possible, what is around us in the moment. Just noticing.
I'm not sure if any of these will remotely apply for the family gathering you are in, but know that you don't have to do it all alone. We are here for you. You can do this. :hug: