I always felt, ever since I was quite young, that there was no real point to planning a life, because I was certain I was going to die ... more accurately, make that be killed, murdered ... and that even carried through to adulthood, when I, more vaguely, would just say, "well, it doesn't matter, I'll never live to be 30" or "I'll never make it to 40".

It is actually a recognized symptom of PTSD - in the DSM IV, they put it this way:

Sense of a foreshortened future (e.g., does not expect to have a career, marriage, children, or a normal lifespan)

The DSM V doesn't make a statement like that - and I feel that that is an unfortunate change, because that sense of nihilism seems "so PTSD" to me -- the closest they come is if you read between the lines of these two:

Persistent (and often distorted) negative beliefs and expectations about oneself or the world (e.g., "I am bad," "The world is completely dangerous").

Markedly diminished interest in (pre-traumatic) significant activities.

I always felt that way, and because of that, I underachieved according to my true potential. Which is just another thing that was stolen from me.