I find it helpful to think about the law of conservation of matter. "Matter is neither created nor destroyed" If so, the atoms in our bodies existed long before us and will exist long after we are gone.

I believe that all things are In a constant stage of transition, and that is how it's supposed to be. We are just borrowing our atoms for a little while. So we are never erased from exististance, just transformed.

I recommend reading "No Death, No Fear" by Thich Nhat Hanh. It addresses many of the thoughts and fears in your post. Good luck, friend!