I get palpitations every day too, and like cianna, I also go through phases where there are more and then there are less. Right now I am in the MORE phase. They scare me too, a ton. Sometimes I get 3 a minute. And like you, I am scared that I will some day die of an undiagnosed heart issue. In fact, today I had an emotional breakdown and cried for hours about these darn things.
The best thing to do is to find a support network, whether that is friends or family or a counselor is up to you. Have someone you can talk to when you are having who can talk some sense to you when you are feeling defeated. I use my husband in this situation, I cry, pout, and explain how scared I am, and he reinforces the doctor's wisdom that they are harmless.
It is so hard. I really feel for you on this. I am going through the same troubles. :(