Poor you FAB. I am not aware of your back story, and wouldn't like to label you in any way. I just want to give you a {{{}}} because it sounds like what you really need right now (though the tough love you are getting here also makes sense tbh).
I am a lifelong hypochondriac (not saying you are one too) and certainly know what it is to have anxiety about symptoms. You have to try to rationalise what you feel a bit. EG, last night you had symptoms that scared you relating to your heart. Yet today, here you are, still with us (thankfully). Now doesnt that tell you that, at the very least, whatever you suffered last night was not as serious as it appeared to be at the time? If you can think yourself into a panic, you can think yourself out of one too. Also, if your GP has sent you away with nothing before, it is possible this means there was nothing serious wrong with you. I know you say previous doctors have let you down in the past - I have experienced that on a major level too and my trust in doctors is reduced - but, possibly, this doctor was a good one and could see your symptoms were nothing to worry about.
I found a good GP this spring who took weeks to go through all my symptoms with me, did all the tests, and eventually convinced me that I was not dying of one thing, but actually had something else - quite trivial, and definitely related to psychology rather than physiology. Since then, my symptoms have decreased.
Oh I am rambling on now. I really just wanted to give you the hug and say I sort of understood where you are coming from when it comes to worrying about symptoms.