Hey there, Floating On! I gotta say, that sort of anxiety sounds absolutely awful to deal with on a regular basis. If I experienced something like that every time I found myself in a social situation, I'd probably never go outside again! And I can definitely see why you'd want to find some strategies for dealing with it, given that avoiding people all the time...doesn't really work.
I don't have severe social anxiety in general, so my understanding of what you experience is likely severely limited, but I do experience some situational forms of it that may give me a window. In my case, I'm very much a social hermit and tend to avoid people in general, not out of anxiety most of the time, but just because I prefer keeping to myself or the company of those few who really understand me, and also because I generally don't feel I have a lot to offer or want to talk much.
That said, I have experienced some fairly intense anxiety in certain cases, primarily when I've avoided someone or put them off for a long time. Case in point, my mom and my brother, who live together about an hour and a hall's drive away from me. I should see them far, far more often than I do, but I don't. And if I don't see or talk to them in any substantial manner for a long time, the guilt I feel about that tends to make me isolate myself even more. I feel like I can't reach out, visit, or get in contact again without somehow resolving or explaining why I cut myself off in the first place. Perhaps you may have some experience with this. In any case, it makes even so much as answering a text (or, more likely, 'not' answering it until much later when they call repeatedly just to verify I'm alive) a very stressful affair.
I have managed to find a solution, though. It's situational, and it only works some of the time, but maybe it'll be useful to you. I find that the narrative I hold regarding social contact makes a huge difference in how I respond to it emotionally. To make a long story short, I can choose to view it in one of two ways: something I have to do, or something I choose to do. In the case of the former, it could be something like 'I need to see this person because I've been avoiding them for a long time'. Or it could be 'I don't really want to go, but they need me, or it's expected of me.' This narrative treats a social occasion as a 'duty' of some sort, something I don't really want to be doing, but feel forced into. And when it's a duty, that's when it's hard, and when I get anxious.
But if I can create a different narrative, one in which a social encounter is something I want and choose for myself, it's different. Maybe there's a movie I want to see with my family. Maybe I want to bring presents, or cook a new recipe for them I've really been wanting to try. I try to get myself excited, make plans, suggest an outing, anything that makes it seem like I'm doing something I want to, and that I'm invested in. And very often, it works.
I realize that this in and of itself could be considered a form of rumination. Thinking about the encounter beforehand, making plans for it, it's similar in nature. But maybe that's a good thing. If you're going to be thinking about it in advance anyway, perhaps changing the narrative tone in which you think about it from one of dread to one of excitement and anticipation can help. Have you ever tried something similar, and if so, has it worked for you?