Pounding heart/pulse sounds like it could very well be anxiety related if they checked heart function, blood chemistry, blood pressure and the other physical causes then you need to talk to them about how to control the anxiety.
From what I have learned there are number tools, both medical and behaviorial. It is somewhat trial and error and home work to find out which one works best for you. It is painful, but all of the long term solutions seem to take time.
Please accept my empathy. I have felt lost and hopeless too, but if you keep talking to your doctor about what works and what doesn't and you work on alternate techniques of coping with stress it will get better.
Please accept my very sincere wishes that you are feeling better when you read this. And try not to focus on the symptoms. When I get sucked into that cycle it only feeds on its self. Its not easy, but try to find a distraction.
good luck and best wishes.
