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By Elisha Fieldstadt and The Associated Press

Many of the thousand people who gathered at Ground Zero in New York on Sunday were there for the 15th time — and even though a decade and a half has passed since the Sept. 11 attacks, many say the grief and the feeling of horror that befell the country that day will linger forever.

"It doesn't get easier. The grief never goes away. You don't move forward — it always stays with you," Tom Acquaviva, of Wayne, New Jersey, who lost his son Paul Acquaviva, said at the annual 9/11 memorial service at Ground Zero in New York.

PHOTOS: America Marks 15th Anniversary of 9/11 Terror Attacks

Andrew Card, who was President George W. Bush's chief of staff on Sept. 11, 2001 and was responsible for delivering the news of the attacks to the president, was also inevitably forever affected, but he acknowledges that the lives of each and every person was altered that day.

"That day changed all of us," Card told NBC News. "It changed America. And it changed the world."

Granvilette Kestenbaum, who lost her astrophysicist husband, Howard Kestenbaum in the attacks, said during the New York memorial Sunday that while the nation and world so often feel divided, the shared sense of a day that touched everyone still unites people of different religions, political beliefs, and nationalities.

"The things we think separate us really don't. We're all part of this one Earth in this vast universe," Kestenbaum said. "We're all ordinary, and we're all special, we're all connected. We waste precious time by thinking otherwise."

James Johnson, a retired New York City police sergeant, who worked on the rescue and recovery efforts in early 2002, said he feels the yearning for closure is what binds everyone who can remember the attacks.

"I've got mixed emotions, but I'm still kind of numb," he said at the memorial. "I think everyone needs closure, and this is my time to have closure."

Some, though, say they may never attain complete closure.

Among those are the families of the 1,113 victims of the World Trade Center bombings who do not have physical remains of the dead. Theirs have never been found.

The void is like a ghost, "intangible, lingering, the opposite of something that is concrete and tangible," said Chip Colwell, senior curator of anthropology at the Denver Museum of Nature & Science who has advised families of 9/11 victims. "That leads to ongoing pain and suffering for many of these families."

"There's an unreality. There's a feeling you can't accept what happened because you have no tangible truth of what happened," said Sally Regenhard, who doesn't have a grave to visit because the remains of her son Christian Regenhard, were never recovered. "It's a pain in your heart that you constantly live with," she said.

Cathy Cava, who lost her sister, Grace Susca Galante, seeks that ability to accept by visiting the site of Ground Zero. She has attended the 9/11 ceremony each year for the past 15.

"I will keep coming as long as I am walking and breathing," Cava said, wearing a T-shirt with her sister's photo.

"I believe most of her spirit, or at least some of her spirit, is here. I have to think that way."

Nearly 3,000 people died when hijacked planes slammed into the World Trade Center in New York, the Pentagon in Washington, D.C., and a field near Shanksville, Penn., on Sept. 11, 2001. It was the deadliest terror attack on American soil.

President Barack Obama said at a Pentagon memorial service that he is inspired by the resilience of the victims' families.

He quoted Scripture: "Let not steadfast love and faithfulness forsake you. Bind them around your neck. Write them on the table of your heart."

Obama also praised America's diversity and urged Americans not to let their enemies divide them. He called the day "difficult" but one that "reveals the love and faithfulness in your hearts and in the heart of our nation."