I 'discovered' my mortality pretty precisely at the age of 23. I have no idea why. I mean, it sounds ridiculous, 'cause of course I had always known there was a chance I would die. People die. But at 23, I really understood that this actually concerned me, and my life. This was when I was very much still in, so it's a little weird.
As I started fading from the Lie though, it of course became worse. Now not only did I truly realize I could die, I started to realize that I will not only die, but be gone for all eternity (barring some inexplicable miracle).
So.... for a period of time, I didn't sleep well. I would lay down at night to sleep, and listen to my heart, and the pounding of it just got louder and louder. And I couldn't help but think that if it stopped right now, that's it. "Hmm... was that a short stabbing pain? Do I feel a small uneasy feeling in my chest?".
Fortunately it went away. I accepted that I have no choice but to live with death. I'm alive now, that's all I know. My heart is fine. Sure, something can happen, but then I won't be able to control it. It happens if and when it happens.
I don't fear death as such, other than the possible pain and anguish beforehand, but when it's over it's over. What I don't care for is that I won't be able to know how things went for mankind, get more knowledge about the universe, see how life turns out for loved ones, listen to music and so on. To lose life. Then again, it has to end someday anyway, at least in this physical universe. No such thing as 'forever' in this universe. Eventually it will be unsuitable for life.
I don't expect a second chance at life, but if that happens it'll be a pleasant surprise.