Did your doctor show you how to massage your carotid artery?
When I had panic attacks back in the early 80s I worked in the reception of an ER and a doctor showed me how to do this. It lowers your pulse rate.
You stick your two fingers on your carotid artery and feel the pulse. Then you press as hard as you can and massage it (go up and down). I can feel kind of a tickle in my throat when I do this, so I know it’s the correct spot. You may or may not feel this. It works to lower your pulse rate, and it really does work.
Warning: Do NOT try this on both sides of your neck at once, you can actually cause yourself to pass out. If you do one side of your neck that is fine, 'cause the other side will easily compensate for it. But do NOT do both sides at once.
You can also try things to lower a fast pulse rate, like throwing your face into a pan of ice water. This shocks your system. It’s similar to when you slap a hysterical person to get them to calm down.
Another trick to lower pulse is to bear down on your rectum, like your having a bowel movement when you’re constipated. This will also lower pulse rate.
Again, this assumes you are normally healthy and your rapid pulse is because of anxiety or the like. If you have health issues that are causing that you need to get that looked at.
What meds are you on? Beta blockers?