In 1995, as a still-believing-J.W., I said "I am going to grow old in this system of things, and I am going to die in this system of things." So I had some time to adjust to that kind of thinking.
But it was different when I realized that death is the end of it all. That's not just because such a thought is disturbing, but it is because I realized that as I was also realizing that I was living a lie in a dangerous mind-control cult.
My point is that many of us think heavily about life and if this is all there is, but we only think it's something new. People have been facing mortality as long as there have been people. Thoughts changed as some thought they were going to hell or heaven or Valhalla or were reborn or whatever. But your questions are a major portion of philosophy too.
Your thoughts on leaving a positive impact on lives are great. While nothing we do will last forever, we can make a difference in the temporary spot we hold.