I get the fluttering heart, and I know your frustration. You can feel it and it's totally terrifying, but the doctors insist there is nothing wrong. Although it isn't dangerous, it's horrible. Sorry to hear you are going through this too. I heavily suggest buying a paper bag [like the ones you get on aeroplanes], breathing into it for ten breaths, and breathing out of the bag, normally, for another 15 - this helps to introduce a regular breathing pattern, and it works quite well for me. [but never use a plastic bag!]
I also get a palpatating heart [which I think can lead to dizziness] with excercise, although I am 17, and in good health and shape. My doctor says it's probably because I over anticipate it, and am too aware of the physical symptoms. You'd be suprised the role anxiety can play in everyday life, even when you are not aware that you are infact anxious. It may be worthwhile for you to be assesed for Generalized Anxiety Disorder. Hope this helps. You should be just fine.