> I know that some of this is nerves but it can’t account for all of it.

Why not?

If you've never experienced severe situational anxiety, you may not know how debilitating it can be.

And then, maddeningly, perhaps just minutes after the interview is over, you return to your "normal" self, and start to think about what you would have done in that situation if you weren't nervous.

It's basically stage fright, which can be terrifying if you're not used to the stage.