A great tip I received from one of my doctors is to pretend to be an actor. Just simply pretend to not be nervous, even if your insides feel like they're turning inside out.
Tell yourself that you are Helen Mirren or Leo Dicaprio and you're playing the part of a very calm person going about their day in the next big Scorsese film. You want to nail this next scene because it's the one that's going into the film. Ready ... and ... action!
You don't have to do it for a long time. The goal isn't to be a *good* actor. The goal is just to pretend to be an actor for a minute or two. I know the anxiety doesn't feel good and it scary. But by just pretending to be calm, you will send signals to your brain that will, in turn, send signals to your body that fight or flight isn't needed right now.
You also get the added bonus of shifting your thought away from the anxiety and panic. Why? Because you're brain is using up most of it's CPU to act the part of a calm person.
Anyway - this works for me, when I remember to do it, that is. You should try it. It may work for you too!