Hi Pickle, I could have written this post myself. I too suffered a nervous breakdown (this summer, I'm 28) following the death of both my grandparents. I'd always harboured a fear of death from an early age, much like yourself, but I suppose the many distractions of childhood put it to the back of my mind.
I went to my GP for advice as I'd really hit rock bottom, he put me on Sertraline which really helped with the anxiety and referred me for some counselling. Although I must say the greatest comfort I have found is through the various research I've conducted myself.
I don't know what I 'believe' at the end of the day....I'm not religious but I don't believe that once we die, that's it. I've read so many accounts of childrens' past lives (do a search on here!) as well as near-death experiences, even more recently two quantum physicists theorising that a near-death experience is when the soul leaves the body and enters the universe. I'll find you the link if you're interested.
I don't know if any of this is true, but it gives me comfort. And even if it isn't true, even if there's 'nothingness' I seek comfort in the fact that we're all in this together, no one is immortal.
I hope you find the peace of mind you need to get you through, and I'm here anytime you want to talk about it.