Hi Hopeful,
Instead of taking quick breath, try to ask yourself a question if you can in the moment the panic is about to begin. I know that once it's in full swing, it may not be possible to reason with yourself, but if you can catch it early enough... The question you want to ask is, "What is exactly upsetting me right now?" There could be 2 types of answers that your mind may serve you:
1. It may struggle with an answer because technically there isn't anything in particular that should be upsetting you, so that in and of itself is likely to diffuse the tension.
2. It may give you the answer such as, "There are all these customers, and I'm afraid I won't be able to keep up and provide good service". In this case, you can follow up with a question, "Do I have control over it?' Obviously, you don't have control over the number of customers, so whatever you don't have control over, try to let go. Whatever you do have control over (you providing good service), you can say "I'll do the best that I can, and the rest I don't have control over, so I'm just going to let it go"
It may or may not work, because I've learned that everyone's anxiety and panic is unique in a way, so what works for one person may not for another.
Good luck and happy holidays!