Re: can't do this anymore
It's okay not to say hi to your former coworkers. People walk around all the time wrapped up in their own little worlds, and they don't even have anxiety (or not show it).
Was there eye contact? Why do you feel like you "should" say hi? You could also frame it as, did your former coworker(s) say hi? If not, then that person might have anxiety, too. Or just feel like they don't know you well enough to say hi, or not recognize you anymore.
It's okay to have boundaries and personal space. Other people do, too, and they change all the time depending on the situation. It's okay to just be yourself, and even be perceived as rude or something. You might say hi one day, and just walk on by the next day.
I was standing outside a place I worked at, and someone came up and asked if I remembered working with him there. I didn't, and had to admit it, and it wasn't the end of the world, even though I felt uncomfortable and he looked disappointed. Life goes on.