It's interesting how many different types of issues the label social anxiety covers. That's one of the reasons why certain things work for some but not for others.
Exposure can be useful but you have to make sure that you don't overdo it.
Have you tried any gradual/graded/systematic desensitization? It can be done as real or imagined. Here's a link that talks about imagined version and has a lot of useful extra notes to improve the chances of it working. http://www.guidetopsychology.com/sysden.htm
I've heard the following statement but didn't find it matching my experience.
All fear comes from dwelling on the past or worrying about the future. You don’t experience fear when you are in the present.
A recent book I read, mentioned these ways to become anxious.
1. Danger is present or near, in time or might be coming in the future
2. Internal trigger stimulus (body sensation)
3. Triggered by thoughts or memories
4. Thoughts or memories (existential dread) about meaningful life, inevitability of death, difficulty of making decisions that have moral value.
If someone suddenly threatens you with a knife or gun, that fear is about as much being in the present as it can be. Most of my issues were also like that - (e.g. standing in line or giving speed, etc) as my mind went blank often in the higher intensity panicky situations. An improv class illustrated it really well. When I had a mind block, our teacher said "Don't worry about anything. Just say the first thing that comes to mind". I wish that had been my situation ... but I couldn't think of anything. That mind block happened quite a few times, but there were somewhat more times where I was able to come up with something and make people laugh.
Sometimes attention diversion techniques can work but if the fear intensity is too high, then it's better to focus on desensitizing the fear trigger (as mentioned above) with other approaches.