Dear Quilt:-
To put my psychologist's hat on (I am not a psychologist but I am 34 and have experienced a wide range of mental health challenges in my lifetime!) have you had a relative die recently? It is very common for young people to feel invincible up to the point where a grandmother/grandfather or someone else close dies and then all of a sudden it makes you aware of your own mortality!
Either that - or are you fixating too much on celebrities who have died young (Peaches Geldof/Amy Winehouse/Nirvan etc etc?)
Or...the bit about the heart sensations - to the naive/worried/anxious a palpitation/missed beat can feel like an imminent fatal heart problem! I suffered from panic attacks in my early-mid 20s (although very rarely-never have them now!) - once I went into the new Bull Ring in Birmingham and the change in heat/humidity caused by the glass made me convinced I was falling ill - I panicked to the point I had to be carried out shaking by friends and an ambulance was called! Only later did I get diagnosed with panic attacks! I found the best way around them was to keep myself busy (they were always worse in holidays/weekends than at work) and, bizarrely to accept there IS a finite chance I could just, randomly drop dead at any point but I shouldn't worry about it! If you've never had one and end up getting one (touch wood you won't) it will feel like you can't breathe and your heart will race - which, Catch 22 style makes the symptoms worse until your body just naturally calms itself down - 20 min after even a bad panic attack you generally feel fine!
As long as your death thoughts stay at a dull roar (or better fade) and don;t interfere with your everyday life, keep calm and carry on! If they do, then see your GP. Don't be scared about what they may think - you won;t be institutionalised or anything! They will be really sympathetic and trust me - GPS have heard it ALL before AND far worse so your case will not be unusual!
Good luck!