In some years, the images and memories are faded, as if from long, long ago. Other years, the horrors feel as fresh as when the burning towers collapsed.

Yet few anniversaries of 9/11 have been as fraught with troubling emotions as today’s. From the vantage point of the 18th anniversary, it is unfortunately true that the worst day in American history forged the last great moment of national unity.

The mourning and sense of common purpose that were so distinct then seem as if they happened in a different country in another century. Now our nation is not just polarized. It is fractured.

“I’ve never seen it like this,” an elderly friend told me recently. “I’m afraid of what’s happening to our country.”

She is not alone. And while the 9/11 attack certainly didn’t cause our bitter divisions, its ramifications are among the powerful forces still shaping our dangerous world.

President Trump’s decision to call off peace talks with the Taliban after a suicide bomber killed an American soldier last week reminds us that the Afghan terror group that gave safe harbor to Osama bin Laden is still as evil as ever.

The death of Sgt. 1st Class Elis Barreto Ortiz brings to nearly 2,400 the number of our service members who have died in Afghanistan since October of 2001. More than 20,000 have been wounded.

In Iraq, more than 4,500 US troops have died, with more than 32,000 wounded.

For its part, New York has somber memorials to the dead as well as living reminders of the undaunted courage we witnessed on that awful day. The Post reports that 13 children of firefighters who died trying to save others are members of the FDNY Academy class that will graduate in two weeks.

The “legacy” class also includes the son of a police officer killed on 9/11 and the sons of two firefighters who died of illnesses linked to their recovery work at Ground Zero.

“Bravery runs in these extraordinary families,” Fire Commissioner Daniel Nigro said.

A brother of one of the probies put it this way: “There are no negatives. We know the risks. We always liked helping people.”

Those brave, straightforward words echo the eulogies at the funerals that dominated the city for weeks in the aftermath. Survivors and loved ones told the stories of people who had the extraordinary courage to run into the burning towers and up the stairs to save others.

Their sacrifice must never be forgotten.

The city came back faster and better than anyone had a right to expect. But now a clear pattern of decline is taking hold.

The blame goes mostly to our absentee mayor , who was feckless even before he abandoned ship in a ridiculous attempt to become president. Only in his juvenile mind is failing at City Hall a credential for the White House.

Meanwhile, the quality of life is taking a sharp turn for the worse.

The mood of many is captured by reader Maria Smith, whose poignant email I quote at length. She writes:

“I am a native Staten Islander born there before the bridge and have lived in Battery Park City for 30 years, from where I witnessed and survived both the 1993 bombings and the 9/11 attack.

“I chose to stay in lower Manhattan to rebuild my neighborhood at a time when the real estate value of my home was ‘zero.’

“Half of Battery Park City moved out after 9/11 but I, along with many others, and especially Community Board 1, fought to rebuild our neighborhood. It wasn’t easy.

“We lived with the National Guard for six months; Con Ed and Verizon wiring lay on top of the sidewalks for about a year. Many stores closed and never reopened.

“I write all this on the eve of 9/11 because I am looking at my city now and am horrified. The streets are filthy. The cops can’t do anything. The homeless situation is beyond tragic. And there is a hostility and violence in the air that I have not felt since I moved to Manhattan 30 years ago.

“To be honest, walking to work today, I thought to myself, ‘Why did I work so hard and choose to fight to stay here?’

“This is what my city has become: a place for people who are not from New York, who are either extremely wealthy or extremely poor. A city where the rules no longer apply. A city where there is once again graffiti. I can’t believe it.

“Yet no one says anything about how bad things are and it appears as though none of our political leaders have the courage or guts to take on the lack of leadership of this mayor.

“I write out of frustration, out of concern, out of fear. Like so many others have done, I’m traveling to North Carolina at the end of the month to see what’s available in terms of jobs and housing.

“I never thought I would say that. My family goes back to the 1840s in Brooklyn due to the Irish famine. We are real New Yorkers and one by one we are leaving.

“I am usually upbeat about my home, my city, but this summer turned out to be a tipping point.”

There is no argument or even a quibble over a single thing she says. Eighteen years after its worst day, the city faces a new crisis. This one is self-inflicted.

One in a billion

A fascinating Wall Street Journal piece on how geologists are finding new details in the Gulf of Mexico about the giant asteroid that wiped out the dinosaurs and much of life on Earth 65 million years ago includes this reassuring paragraph.

“There are currently no sizeable asteroids known to be on a collision course with Earth, NASA astronomers say. The dinosaur-killing asteroid, estimated to have been up to 50 miles or so across, was likely an event that occurs only once every billion years, the scientists said.”

Whew, one less thing to worry about.

Acosta’s hit a ‘mess’

Poor Jim Acosta. The CNN reporter’s act is so tired, he can’t even get the White House angry anymore.

After John Bolton’s firing, Acosta asked, “Is this national security team a mess?” In a rapid response, Treasury Secretary Steven Mnuchin said, “Absolutely not,” then added, “That’s the most ridiculous question I’ve ever heard of.”

Touché.

Demwit don foes

Is the House Judiciary Committee investigating the president for possible impeachment? The answer depends on who’s talking.

That’s the gist of a Politico story that says Chairman Jerry Nadler’s team is lost in a “messaging muddle” about whether it has opened a formal impeachment process. Committee members aren’t sure what to say and staff members are fighting with each other over terminology. Speaker Nancy Pelosi says only the party is united.

And they say Trump is incompetent.