I see that you are an audio therapy series user. A follow up question would be, are you doing the therapy. You don't need to answer that. Only you will know if you have done it and done it consistently and persistently. I say this because I have to answer from a perspective of therapy and solutions, so the assumptions of my answers go to the therapy.
I'm not here to doubt what you say, and especially to doubt the intensity of the feelings you describe about what you say. There are many social anxiety sufferers, some of my friends in past groups, who specifically have problems with stronger physical sensations or symptoms, such as feeling of muscle/neck twitching, feeling frozen and not being able to react physically or especially react what they deem appropriately with facial expressions because they feel their fact completely freezes over. It's nothing to make light of - as anyone with these stronger physical issues will understand. It sort of clouds out everything else.
Did I notice that these people had the symptoms they most horribly felt they did? In some cases I did not. In other cases, yes, I could tell when some talked about being unable to react and when they could never smile or move their faces to smile. Yes, I could see that. I keep in touch with some of those people now, specifically my "frozen face" friend, and the way for him forward was not to dissect his frozen face and take care of that alone. He continued with the cognitive therapy. It's a boring answer for you. It also took him longer to internalize the cognitive and therefore the behavioral, so that he could calm down, loosen up and truly believe in the feeling of relaxing and accepting/not resisting, and therefore becoming less worried about that, so that the physical problem this was... lessened and lessened.
Of course your confidence will be affected. Setbacks are no fun for anyone and we don't want them. And we read about that possibility but we secretly hope that setbacks are for everyone else and not US. Too bad, because setbacks are a part of the program. Your ANTs thoughts, or really those old neural pathways are still working to hang on in your brain and still competing for a while, and sometimes we still slide down those and that sometimes leads to a huge setback that hits us way off course. And so, all of us must assume that a setback will come, and we must keep repeating that also: "Setbacks are a sign of progress". SA doesn't want to let you just get away so easily. And it will keep you if you let this balloon beyond the basics of therapy - go back and do what's right in all ANT situations. Slow down, be conditional, don't resist, don't accept ANT thinking, reinforce the new neutral and the new positive.
You are still speeding up and getting tight. You are still fighting this from happening. You are still judging yourself as a serial killer and inferior because this happens, imagining what others think and rejecting yourself, and thus continuing the cycle that this is a strong neural pathway that can continued to be used by ANTs. So my answer is not sexy, but I don't intend to make sexy answers to people to lie to you.
Don't let the setback throw you off. Go back to the therapy, back farther than you may have been and consistently and persistently reinforce again, and show that pathway that you are still choosing calmly to allow the new pathway to exist as a possibility, a possibility that will grow stronger in time that will pave the way for you slowing down, not tying everything onto this issue, accepting yourself no matter what.
Again, it's not easy. Setbacks and big painful ones like this make us want to completely give up. And that's exactly when you should double down and continue on, as much as you don't feel it. My first setback had me doubting the whole thing. I just kept repeating that statement "setbacks are a sign of progress". I didn't believe it at first, until finally I did.