Theresa May hates change, while governing in an age of upheaval. It is an unfortunate combination: inflexible temperament meets volatile circumstance. The result is periods of tense stasis punctuated by spasmodic crises. This is a chronic condition afflicting the government and Amber Rudd’s resignation on Sunday is the latest symptom. That doesn’t exonerate the former home secretary of blame for her political ruin.

She misled parliament over targets for deporting illegal immigrants. She then pleaded ignorance, but in terms that proved to be less than candid. But at the root of Rudd’s problem was the impossibility of doing her boss’s old job differently to the way her boss had done it. She was one of the few ministers from David Cameron’s cabinet to survive May’s 2016 purge of the pre-referendum Tory ancien regime. The condition of her elevation was that she leave her liberal, remain-voting impulses at the door.

Timeline Amber Rudd's apologies Show Hide The home secretary has issued five apologies in the last week – four of them over her department's handling of the Windrush crisis and immigration targets. Rudd delivered an unprecedented apology to parliament and acknowledged that her department had “lost sight of individuals” and become “too concerned with policy”. Rudd apologised for failing to grasp the scale of the problem. She told the home affairs select committee: “I bitterly, deeply regret that I didn’t see it as more than individual cases gone wrong that needed addressing. I didn’t see it as a systemic issue until very recently.” On Thursday morning, Rudd was forced to admit officials did have targets for removals, having previously denied their existence. “The immigration arm of the Home Office has been using local targets for internal performance management. These were not published targets against which performance was assessed, but if they were used inappropriately then I am clear that this will have to change." On Thursday afternoon, Rudd was forced to issue a hasty clarification after appearing to leave the door open to the UK staying in a customs union with the EU. “I should have been clearer – of course when we leave the EU we will be leaving the customs union." In a series of late-night tweets, Rudd apologised for not being aware of documents, leaked to the Guardian, which set out immigration removal targets.

‘I wasn’t aware of specific removal targets. I accept I should have been and I’m sorry that I wasn’t. I didn’t see the leaked document, although it was copied to my office as many documents are."

Two months later, Rudd was talking about forcing companies to list the number of foreigners they employ. The theory was that this initiative would promote a Britons-first hiring culture. It was a nasty idea that didn’t survive much media interrogation, but it hung in the air long enough to disabuse anyone of the hope that culture change was coming to the Home Office.

A Tory MP once told me that May had chosen the most poisonous chalice for Rudd to sip at the cabinet table in order to break her spirit. Her energy would be harnessed to the consolidation of the prime minister’s Home Office legacy, while any ambitions she might have to copy May’s route to No 10, perhaps as the champion of a liberal Tory faction, would be burned up in the process.

That struck me as paranoid at the time (although if there was such a plan, it appears to have worked). A former Downing Street aide reports a similar calculation behind the placement of Boris Johnson in the Foreign Office. In that case, May was exploiting not diligence but its opposite. Johnson’s ambitions to be leader would be hobbled by the demands of a serious job, which would expose his congenital unseriousness. (That, too, appears to be working.)

These days, May has less leeway when hiring. In the absence of a parliamentary majority, she can’t afford to make new enemies or alienate any Tory faction. Given those constraints, the promotion of Sajid Javid to fill the vacancy left by Rudd looks shrewd. He was a non-committal remainer who embraced Brexit without a backwards glance. As the son of Pakistani immigrants, he also represents a pioneering presence at the top of government.

The prime minister tries to sound thrilled by the future, but her political body language cries out for the past

What May expects to change as a result of Javid’s appointment is hard to know. Decrypting the prime minister’s motivations is an unrewarding pastime. Even long-serving colleagues find her inscrutable. A discernible trend is that she builds fortifications, not bridges. This is true of personnel and policy. She keeps a tight circle of absolute loyalists around her and engages in minimal diplomacy. Likewise, with Brexit, she talks about the need for a “deep and special partnership” with the rest of Europe, but her primary instincts are more parochial and insular.

When May was new to the job, popular, with a blank canvas on which to paint her priorities, she depicted Brexit not in terms of new alignments with continental neighbours but as an expression of domestic anger against metropolitan elites. She defined herself as the champion of ordinary patriots against haughty, globalised “citizens of nowhere”.

May is not a wild ethnic nationalist, itching to persecute those who look different. As home secretary she denounced police abuses of stop-and-search powers, too often deployed to harass young black men. As prime minister she has spoken with persuasive urgency about the way racism obstructs many pathways to social mobility. And yet she is also the architect of the “hostile environment” policy that gave officials licence to treat thousands of entirely blameless black British citizens as criminals and aliens.

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The explicit fixation that led the prime minister into this moral quagmire is not race, but borders and their control. That is how the targets and crackdowns began. The view that frontiers should be policed is uncontroversial. But that is different from the cult of numerical precision and the fantasy of counting everyone in and out. That notion is fused with white-majority nostalgia, inseparable from the myth of the overcrowded island nation whose hospitality has been abused. And that often comes as a set with a gut feeling that national decline and racial diversity are somehow correlated. Those are common prejudices and I suspect they inform May’s conservatism more than she admits, even to herself.

The prime minister tries to sound thrilled by the future, but all the while her political body language cries out for the past. When talking about industrial strategy, she praises digital innovation. But her chief policy interest in the internet has always been how better to control it when terrorists and paedophiles thrive in its ungoverned recesses.

She says the things exuberant Brexiteers want to hear about the opportunities for Britain beyond the confines of the European Union. But she negotiates as if lost in a forest of terrible options, feeling her way to the path of greatest continuity. She is holding out for a way to change everything while keeping things the same, handling each crisis as it comes without dealing with the underlying problems, and so sowing the seeds of the next crisis.

Eventually the crisis will come that overwhelms her completely. But until then, May’s awkward political destiny is set: to be resisting and leading change at the same time; to be forced out into the world when she would rather stay at home; and to march reluctantly to a revolutionary drum when she looks as if she would rather be standing still.

• Rafael Behr is a Guardian columnist