Take a deep breath and accept the fact that you're going to die some day.
Its not fun but its inevitable and thinking too much about dieing is not healthy for you. As you can feel. Really, theres no need to worry about it..
If its getting so bad that you get anxiety attacks, you might want to go see professional because they can teach you how to overcome that and maybe they give some meds if it gets very bad. In the end you just have to accept the fact that everyone dies at some point.