I agree with @zenvelo‘s suggestion to try controlled breathing, though slightly different:
-Try to breath with your abdomen, not your chest. This is a more natural way of breathing (just look at a baby or even a dog napping and you’ll see)
-Breath in through your nose for two seconds, expanding your abdomen
-Breath out through your mouth for four seconds, slowing allowing your abdomen to shrink
Repeat for as long as you need to slow your heartbeat (and your racing thoughts!).
My suggestion is to practice this technique when you’re not anxious, somewhere between 1–3 times a day for 5–10 minutes. One day, when you need to calm down, this relaxed breathing will come naturally because you’ve had practice with it.