Your focus becomes your reality. First; examine your belief system, or lack of one. The computer you are using right now, works via the electron tunnelling effect of quantum physics. Even if you maintain no religious beliefs whatsoever; there is another way of perceiving reality, including personal mortality, through the use of quantum physics, in which the direct implication is that for every event which occurs in this universe, at a particular time; in other universes, which "split off" from this one, every tiny fraction of a second. So, in other universes, you are already on your deathbed, or long buried, and in others, the "other you's" will continue to live on, and in others, you are just being born. Now, all those others are at least very slightly different from this you, some very much so, and others unrecognisably different.

There are many implications involved in this, particularly when you "run back the clock", and consider exactly how you came to be here in the first place. Learn how we evolved from simple, poorly self replicating molecules, most of which were created in stars, many light years away, more than 5 billion years ago. We share a common ancestor with not only animals, birds, and fish, but also trees, fungi, and bacteria. This is largely a world of single celled creatures, (prokaryotes) certainly in number, and by weight. They don't die, in the sense that animals do. Their cultures live on. Because of the way our senses work, we tend to feel isolated in our bodies, seperated from the thoughts, feelings, and experiences of others. We therefore consider the end of our own life to be the end of everything, when in fact, it is but the end of one tiny facet of the totality of life on this planet.

 Because we have evolved from animals, we have inhereted a survival instinct, and to help us through the many dangerous situations which abounded in times long past, our distant ancestors were evolutionarily selected for the massive boost of speed and power created by the release of adrenaline, in the "fight, or flight" response of a panic attack. In a similar manner to the single celled organisms, our culture will live on, too. The implication of this is that we; all of us get to experience infinite trillions of lives; each one slightly different than the other; some starting from the same position, and then diverging, or branching in different ways. Of course, there is no way to ever prove this. Ordinarily, when regarding death, our mind perceives an end to existence, and immediately responds in its built in, primitive way, treating it exactly the same as an attack from a bear, or lion.

 So, you have two options: One is to do what I have done, and keep on thinking about death, and what it really means, so that eventually your panic reaction is no longer triggered by such thoughts, or to avoid considering your inevitable demise.  Most people are suggestible, to some degree, so you could either seek professional hypnotherapy. More advice, some from the ancients: "Keep death on your left shoulder". Use it to find focus, clarity, and motivation in your life. Decide on what it is that you want to accomplish in your time here. Make a workable plan, divided into readily acheivable stages, and put it into action. Your other choice is to continue to live in fear, never truly enjoying life for the roller coaster ride that it can be; ending up in the same place, but without taking the scenic tour, and being carsick all the way.