As was mentioned, I've got two main characters for Honor At Stake.
As I've also mentioned, they are both New Yorkers ... well, the vampire has lived in my fair city for about 25 years, so that confers some sort of status of citizenship.
Which means that both of them were in town when 9-11 happened.
The human was about five or six, and got to watch it live, and in person, in the borough across the river, Brooklyn. Yup. My character watched the whole thing happen live. Even remembers the snow on the ground that wasn't even snow. It was ash.
The vampire remembers 9-11 just as well, but couldn't go outside until the towers fell. The ash and dust blocked out the sun, and, well, a vampire didn't really need to breathe in all of that mess, so could go outside at that point.
But before that, the vampire could see the towers fall over and over again on tv, because cable made it possible to watch the second plane strike over, and over and over again, because this was before everyone having an iPhone, and we didn't see the first hit until later.
One character could only barely understand that an airplane had hit buildings, and that people were falling out of the windows — couldn't hear the impact, but a young imagination is more than enough to fill in that particular blank.
The vampire understood what was happening perfectly well, and wondered how many thousands dead wouldn't be reported, because vampires went up in flame and fire and sunlight from broken windows.
One would grow up to become a monster. The other wished that vampires were bigger monsters -- and hungry for Islamofascists.
You see, I won't say that the world changed for me on 9-11. I had always known that Islamic terrorists needed to either stay home and kill each other, let the Israelis kill them, or be killed by us. I always knew that blowing up some random stuff when they did something stupid was a bad idea — they knew what we would do, where we would bomb, and suffered not at all for what we did with a few bombs. I had always known that Osama bin Laden would have been better off six feet under.
Before 9-11, I had figured that Islamofascism would die a slow death, along the line of Darwin. Yes, as I stated before, I would have been happy to have offed Saddam in the 90s, or invade Sudan to stop the ongoing genocide, but I figured no one would do it for purely humanitarian reasons.
After 9-11? Kill 'em all.
Will it follow me and all of my characters for all the days of my life? Maybe. I honestly don't know. It's possible. Codename: Winterborn would not have happened without it. And I can honestly say that Honor at Stake wouldn't have happened without it. Not only has it affected me, and the characters, but it shaped the story the first time out.
The first iteration of the story, back when it was fan fiction, 9-11 became part of the story. I had to rewrite it when the events came up in Demons are Forever.
I have a space opera that I wrote back in the 90s that has undergone some serious revision as far as the bad guys are concerned. It's not entirely influenced by the evil of 9-11, but there are echoes and elements here and there.
Victor Davis Hansen, in his book Ripples of Battle, examined how small battles -- some that people never even heard of -- have lasting effects, touching all sorts of things. With 9-11, it might well have created the next generation of politicians out of vets from the sandbox. Maybe you have a few people who lost someone, and they become a President of the United States who nukes Mecca. No idea.
But I know that 9-11 has definitely shaped these books, and my characters, and large parts of the way I think and approach things.
And, right now, I also know that I will never forget. I don't think my brain will let me.
I will not forget
I remember when I first heard about it Dad and I were rounding the corner to where I work
On the evening of that day, I found the full lyrics of the Battle Hymn of the Republic, and wrote them on a sheet of paper with the American flag as background. At the bottom I wrote "9/11/2001 -- Never Forgive".
That wording was a deliberate choice; I meant it then and still do.
You might look for the full lyrics; the first verse is well known and a good one but several of the others are even better.