(CNN) The abrupt resignation of Department of Homeland Security Kirstjen Nielsen ends a tumultuous two-ish year tenure in the job in which one simple fact just kept popping up: No one on earth could do this job to the satisfaction of President Donald Trump.

The reporting coming out of Nielsen's resignation speaks to the impossibility of the DHS job under Trump.

"Trump was furious with Nielsen [in May 2018], telling her he didn't think she was doing enough to secure the border. But Nielsen stood her ground, citing the law in certain instances, the source said.

"Nielsen increasingly pushed back when the President lashed out at her department for not doing more to stem the tide of undocumented immigrants.

"A senior administration official said that in recent days Trump and Nielsen had again clashed over the issue. He accused her of not doing her job, and she responded forcefully."

"The president called Ms. Nielsen at home early in the mornings to demand that she take action to stop migrants from entering the country, including doing things that were clearly illegal, such as blocking all migrants from seeking asylum. She repeatedly noted the limitations imposed on her department by federal laws, court settlements and international obligations.

"Those responses only infuriated Mr. Trump further. The president's fury erupted in the spring of 2018 as Ms. Nielsen hesitated for weeks about whether to sign a memo ordering the routine separation of migrant children from their families so that the parents could be detained."

There's a few things to think about here:

1) Trump believes to his core -- and he's not wrong -- that if there was one reason he won in 2016 it was because of his pledge to a) build a wall along our southern border and b) broadly toughen America's immigration policies. If there is ANY campaign promise that he is absolutely and totally committed to keeping, it's to be the toughest president ever when it comes to the border. One needs look no further than Trump's decision -- against his own party's wishes -- to declare a state of emergency on the southern border in order to re-appropriate money to build his wall. Or his push behind the scenes, reported by CNN Monday, to reinstate the policy of family separation at the border that led to a slew of very young children taken from their parents without any real way to track their whereabouts.

2) Trump has very little understanding of -- or care for -- boundaries of any sort. And that goes double for immigration and border issues because of everything I mentioned above. This is his issue. This is the issue his people care about. And he'll be damned if he's told by some bureaucrat that he can't do what he wants on the border. That Trump would instruct Nielsen to block all migrants from entering the country and then blame her when she told him that was, in a word, illegal, speaks to the detachment from reality that President has on this issue.

Combine those two factors and you get this, from a senior administration official on Nielsen's thinking : "[She] believed the situation was becoming untenable ... [with the President] becoming increasingly unhinged about the border crisis and making unreasonable and even impossible requests."

Think of this in your own life. Your boss calls you in. She tells you to go find a Woolly Mammoth -- and to make it snappy. (You are an assistant zookeeper in this scenario.) When you come back to her empty-handed and explain that the Woolly Mammoth went extinct around 1700 BC, she tells you that you are not working hard enough to find one. And that if she was in your job, she would have found a Woolly Mammoth hours ago. And that everyone knows that you can find a Woolly Mammoth if you look hard enough.

Look. There are a LOT of very tough jobs in the Trump administration: chief of staff, communications director and press secretary all jump to mind. But there is NO job more impossible than running the Department of Homeland Security. Nielsen made mistakes, sure. But focusing on her mistakes overlooks the real story here: Donald Trump wants impossible (and, at times, illegal) things done by DHS to address the border crisis. And if you aren't willing to bend (or break) the rules, he thinks you have failed him. And he might think you've failed him anyway; remember that Nielsen enforced a zero-tolerance policy at the border on family separations -- becoming the face of a deeply unpopular policy that Trump eventually walked away from.

In short: It doesn't really matter who Trump names as Nielsen's successor. That person could be the hardest of hard immigration hard-liners. But that person will still be dealing with a Congress less than willing to greenlight some of Trump's more extreme border policies and, well, the rule of law. And no one person can change one of those realities, much less both.