I have been to a couple of my wife's HS reunions with her. Most of the people who showed up were people she never hung out with much in HS either. From what I could see, whoever happened to show up just started socializing with the others, even if they had not been close friends in the past.
So I have a radical proposition for you.
Go to the reunion.
It might the perfect opportunity for someone with social anxiety: you have no emotional investment in these people, and you will probably never see them again. It's a one-shot social occasion with no long-term consequences. Lots of people there will feel as disconnected with others as you will. Nobody is going to focus on you, as if they are testing you. It's not like a "date."
Once you sit down at a table, people around you will start talking, and you will find you can gradually work your way into the conversation, even if only to say a word here and there. Don't worry about anything more than that. So what if people just remember you as "the quiet one at our table?" Enjoy what others are saying, enjoy the food, enjoy the music....and come back home able to say "I did it!"
