I had a pre-depression life. This realization of death came to me then and not after.

I remember it as not being relating to any particular incident or time in my life, it just happened. It was rather shocking to have happen. I just didn't consider death (obviously I knew it would happen...) and then, one day, I had this complete awareness and understanding that there would be a day when I simply wouldn't be here.

That's all I can write about it as it tends to put me into a panic. I'm so sorry you're going through this as it IS terrifying.