Since I first became a mother in 2000, my reality — my “now” — has been my children’s. The babies were an extension of myself, and with no parameters of their own for what is past, present or future, they simply lived in and experienced the world through my perspective.

All of that has changed. The shift has been happening all along, one imperceptible change at a time, but this Sunday was the first time I fully realized the discrepancies between my children’s experiences and my own. It’s a bittersweet moment of marveling in the kids’ independence and mourning a time that I can never get back. It’s something that feels a bit like grief, but turns out to be a lot of responsibility, too.

On Sept. 11, 2001, my first son, Ford, was 10 months old. My husband, Dustin, had been on deployment since Ford was a newborn, and I was looking forward to his homecoming soon.

It was our dog’s birthday, so I was singing “Happy Birthday” to her while Ford giggled in his high chair. I was spoon-feeding him pears. I knew that our nearest relatives were 600 miles away, that it was just “me and Ford,” but so far, I hadn’t felt that alone. I was 24 years old.

My mom called and told me to turn on the television. I watched as the second tower was hit by an airplane.

Later that morning, I made a train track for Ford. He was too young to play with it. He mostly gnawed on the wooden tracks as trails of drool rolled down his chin. But I felt useful and productive as I arranged each piece into what I was telling my laughing baby was “the biggest train track in the world!” All the while, I fought back the reality of how alone we were and how terrified I was by the day’s news.

At some point while I was making the train track, I took a picture of Ford staring out his bedroom window. The memory is so clear to me, I feel like I can reach out and touch his footed pajamas and hear his soft cooing. It is — it was — my “now.”

Fifteen years later, this Sunday, I watched the all-day, as-it-happened coverage of 9/11 on MSNBC. My boys, three of them now and ages 15, 13 and 9, came in and out of the living room and commented on what they saw.

Lindell, 9: Television looked so different back then. The colors are different.

“Back then”? “Different”?

Owen, 13: They really didn’t have much security at airports back then, huh?

There it was again: “Back then.”

But the kicker was when Ford came in, sat down beside me, and watched the historical footage of the Pentagon on fire.

“You were in your exersaucer as I watched this moment in 2001,” I told him. “You chewed on your tiny hand, and I tried not to cry.”

The towers collapsed.

“This is when we got the call that Dad wasn’t coming home, that the deployment was extended. I wanted to go to your grandparents’, but I couldn’t. It was just you and me. I brought your porta-crib into my room that night.”

I showed Ford the picture of him at the bedroom window. He, of course, has no memory of it. It’s just a story from the past, the baby face is foreign. But it’s my lived experience, and the baby is as clear in my mind as the lines on my own face.

It’s all “back then” to Ford. He is a young man now. He’s taller than I am. Smarter, too. He explains things to me about Sept. 11 that I never knew, things he’s read in books or learned at school.

His experience of what we lived through together is textbook. His memory of our months alone together are just stories he’s been told.

But it has always (still) felt like “now” to me.

Except, the colors on the historical footage do look old. And security was lacking. And the world was different. And 15 years had not passed. And my baby is almost grown up. And …

I could almost let despair and grief set in over things like passing youth, children growing up and the world changing. But Sunday night, as I lay in my bed, I thought about all the people who died on Sept. 11, people who never got to experience the simultaneous celebration and sadness felt over one photo of a baby who is now grown-up.

And then I thought of my grandmother, who always talked about the Depression like it was still here or had just happened, and like I had lived it and could remember it, too. And suddenly she made sense. Everything made sense.

And we wake up tomorrow to see what our children’s “now” will be.