Hi there, you poor thing, I know how terrifying heart palpitations can be. I once had one that lasted an hour, I called an ambulance and after about 5 mins of the paramedics being there my heart righted itself and I was fine. I now know the reason it lasted for so long was because I panicked and got myself into a right state. I tend to get palpitations when I am extremely stressed and anxious and if I drink caffeine, it really is a no no for me. The thing that will help is to normalise your palpitations. Once you stop being scared of them, you will reduce the liklihood of having them. Also, our heart is a strong muscle that can well cope with beating fast for a long time - when we were cave people we had to run away from sabre toothed tigers for survival and to also run long distances for hunting (well the men anyway!) so the heart is strong enough to cope with beating fast for ages! What about marathon runners, they go for miles and miles!
I am now not afraid of my palps, I don't like the feeling of having them, but I don't panic. Next time you have one you must lie down and try the 7/11 breathing technique. Put your hand on your abdomen and breathe in deeply through your nose for 7 seconds (or thereabouts), hold it for 3, then breathe out through your mouth giving a 'whooshing' sound for 11 seconds (or thereabouts). The key is to breathe in deeply from your abdomen, not your chest, feel your abdomen rise and fall and to breathe out for longer than you breathe in. Make yourself do this during the palpitation and your heart will just right itself.
Heart Palps are linked very closely to the action of the vegus nerve that runs through the body and is connected to your digestive system. If you eat too much and too fast you are likely to get a palp, also if you are overweight and /or wear tight waistbands.
Another tip is to press cold water onto your eyes lightly with your fingertips whilst having the palpitation, also holding your nose and blowing (like you do when your ears pop on a flight), also coughing can sometimes right the heart. I know these sound mad but my doctor told me about them and the pressing on the eyes one worked straight away for me last time!
Also as others have mentioned you definitely need CBT. There should be someone around you as it is a common form of therapy. Your doctor should be able to refer you or point you in the right direction. You must trust and believe that your heart is strong, nothing is going to happen to you. This has been proved to you by the medical profession, so trust it.
Your thoughts about your palps are what is keeping the cycle going and CBT will address these. For instance if you are thinking 'oh no, here comes another one, what I am going to do, I can't stand it!' then of course, you will panic yourself. But if you think 'here it is again, oh well, I know my heart is strong and healthy, I will breathe and it will pass' then you will feel better.