Mistakes born of arrogance may be the hardest.
In the 2000 presidential campaign, George W. Bush said things that made sense. He indicted President Bill Clinton and previous presidents for "nation-building" - attempting to reshape other countries in America's image.
Once in office, Bush changed his tune. In the aftermath of the terrorist attacks on Sept. 11, 2001, he opted to send America to war against Iraq - even though there was scant evidence that Saddam Hussein had been involved in any way with the 9/11 atrocities or that he possessed the weapons of mass destruction Bush's advisors insisted were hidden in the desert.
We went to war with Iraq, spending billions of dollars, losing many American lives and killing many Iraqis in the process.
We never found the mythical weapons of mass destruction.
And there never was any evidence that Saddam had any part of 9/11.
So then we said the war was about implanting democratic values in Iraq rather than weapons or the attacks on our soil.
In other words, we said it was about nation-building.
In the 2000 presidential campaign, Bush acknowledged that he found it hard to be objective about Iraq because Saddam once had tried to kill Bush's father, President George H. W. Bush.
Barack Obama didn't have that excuse for his lapses in judgment. Even though Obama opposed the war initially, once he was in the White House his policy in regard to Iraq was not nearly as different from George W. Bush's as either Bush or Obama partisans like to pretend.
Both presidents, despite their initial protestations to the contrary, tried their hands at nation-building.
And they failed.
I remember being at a think-tank conference on the East Coast a few years ago where I spent some time with some mid-level career foreign service officers. Over cocktails one evening, I asked them when we might be able to be clear of the Iraq war.
They shook their heads and one said, "Maybe never."
They explained we had gone into the country assuming its people thought like us and wanted the same things Americans do. We presumed that our military and economic power would be enough to remake cultures and societies that had been shaped through thousands of years of history.
We believed, they said, that we could play god - that we could remake a nation with a completely different history and character than ours in our own image.
The belief was as foolish as it was cocky, they argued.
Iraq was going to devolve into a civil war when we left the country. It didn't matter if we stayed 10 years, 20 years or even a century, that civil war was going to come once we left, they said.
The best we could do was delay the tragedy. We weren't going to avoid it.
And we haven't.
The Shiite government and the Sunni insurgents are locked in a brutal struggle that threatens to destabilize the entire region - a region in which the notion of stability itself can be discussed in only the most relative terms.
All of our treasure we've spent and all of the blood we've spilled in Iraq have gone to naught. We now face a country and a region in chaos.
When George W. Bush was running for re-election in 2004 and it had become clear that getting out of Iraq was going to be a lot harder than getting in had been, he defended his decision to go in and stay by saying:
"How do you ask a man to be the last man to die for a mistake?"
That definitely is a hard question to ask, particularly when it might be the truth. But that's also when it's the most important time to ask.
We tried our hand at nation-building in Iraq. A dozen years down the road, it's clear that we failed - and failed in a big way.
This country will be a long time recovering from our attempt to play god in the Middle East. And the families who lost loved ones may never recover.
Some mistakes are hard to get past.
John Krull is director of Franklin College's Pulliam School of Journalism, host of "No Limits" WFYI 90.1 Indianapolis and publisher of TheStatehouseFile.com, a news service powered by Franklin College journalism students and faculty.