What kind of tests have you had? Did your doctor tell you why that your heart speeds up?
It seems like that whenever anyone has a problem with the heart, they end up having anxiety about it happening again. One thing leads to another and before they know it that is all they think about. The anxiety can bring on more and more problems as you know.
You said that your heart doctor could find nothing wrong. Has he not explained anything that makes sense?
I would suggest that you get a complete physical and if everything checks out you should seek a counsler because nobody deserves to live in fear like you are.