Have Hope, your husband's response to the 9/11 anniversary is very much like mine. I did not lose anyone I knew, but it still felt like a horrendous and overwhelmingly horrible attack...and it DID feel very personal (I grew up in NYS, and had many college friends who lived in or near NYC.).
It was a hellishly long day, too. The days that followed were agonizing, and I didn't know anybody who was there (at least, I don't think so)...but I have friends of friends who lost someone.
I personally felt like every neuron in my body had been pulled through a knothole. And I know that the onset of my major depression is largely down to the attacks on that fateful day, because I cannot really get over it.
Your husband's grief is perfectly understandable to me. I have a very hard time watching documentaries about it, but I do it as a kind of exposure therapy. Every fiber of my being reaches out to the souls who were lost, and the families of those people. It must be unimaginable pain.
I think it collectively changed all our neural pathways, I really do.
To me, 21 years has zoomed by, in comparison with the way grief can linger and revisit us.
You have both been through so much. Sending love...