Hi,
I've had the same exact you've been experiencing.
There is a number of things you can calm your heart when it's racing.
1. Wash your face with freezing cold water. Somehow this slows the heart, marine mammals dive in cold water to reduce their heart rates.
2. Breath slowly into a closed bag. Somehow the increased CO2 reduces oxygen due to hyperventilation from breathing too fast (during anxiety).
3. Eat some fresh legumes and vegetables. Bananas and carrots are especially calming to the cardiovascular system. Eat anything that contains a lot of magnesium, its supposed to be good for the heart.
4. Avoid caffeine, cigarettes, alcohol, or any form of sedative or tranquilizer. Avoid medications (except the Xanax you're taking).
5. Relax as much as possible. Given that your doctors have said nothing
is wrong with you, this apparently cannot be bad. Just calm down
when it occurs. Imagine that your heart is slowing down and convince yourself that's happening as well.
I've taken Xanax before, not sure about its long term effects, but its more
appropriate to take it ONLY once when you REALLY need it.