“Sir, can you please have a seat.”

Donald J. Trump had stood up to leave the Manhattan criminal courtroom as Justice Juan M. Merchan was wrapping up a scheduling discussion on Tuesday.

But the judge had not yet adjourned the court or left the bench. Mr. Trump, the 45th president of the United States and the owner of his own company, is used to setting his own pace. Still, when Justice Merchan admonished him to sit back down, the former president did so without saying a word.

The moment underscored a central reality for the presumptive Republican presidential nominee. For the next six weeks, a man who values control and tries to shape environments and outcomes to his will is in control of very little.

Everything about the circumstances in which the former president comes to court every day to sit as the defendant in the People v. Donald J. Trump at 100 Centre Street is repellent to him. The trapped-in-amber surroundings that evoke New York City’s more crime-ridden past. The lack of control. The details of a case in which he is accused of falsifying business records to conceal a payoff to a porn star to keep her claims of an affair with him from emerging in the 2016 election.

Of the four criminal cases Mr. Trump is facing, this is the one that is the most acutely personal. And people close to him are blunt when privately discussing his reaction: He looks around each day and cannot believe he has to be there.

Asked about the former president’s aversion to the case, a campaign spokeswoman, Karoline Leavitt, said that Mr. Trump “proved he will remain defiant” and called the case “political lawfare.”

He is sitting in a decrepit courtroom that, for the second half of last week, was so cold his lead lawyer complained respectfully to the judge about it. Mr. Trump hugged his arms to his chest and told an aide, “It’s freezing.”

For the first few minutes of each day during jury selection, a small pool of still photographers was ushered into Part 59 on the 15th floor of the courthouse. Mr. Trump, obsessed with being seen as strong and being seen generally, prepared for them to rush in front of him by adjusting his suit jacket and contorting his face into a jut-jawed scowl. But, by day’s end on Friday, Mr. Trump appeared haggard and rumpled, his gait off-center, his eyes blank.

Mr. Trump has often seemed to fade into the background in a light wood-paneled room with harsh flourescent lighting and a perpetual smell of sour, coffee-laced breath wafting throughout.

His face has been visible to dozens of reporters watching in an overflow room on a large monitor with a closed-circuit camera trained on the defense table. He has whispered to his lawyer and poked him to get his attention, leafed through sheafs of paper and, at least twice, appeared to nod off during the morning session. (His aides have publicly denied he was dozing.) Nodding off is something that happens from time to time to various people in court proceedings, including jurors, but it conveys, for Mr. Trump, the kind of public vulnerability he has rigorously tried to avoid.

Trials are by nature mundane, with strict routines and long periods of inactivity. Mr. Trump has always steered clear of this type of officialism, whether by eschewing strict schedules or anyone else’s practices or structures, from the time he was in his 20s through his time in the Oval Office.

The mundanity of the courtroom has all but swallowed Mr. Trump, who for decades has sought to project an image of bigness, one he rode from a reality-television studio set to the White House.

When the first panel of 96 prospective jurors was brought into the room last Monday afternoon, Mr. Trump seemed to disappear among them, as they were seated in the jury box and throughout the rows in the well of the court. The judge has made clear that the jurors’ time is his highest priority, even when it comes at the former president’s expense.

Mr. Trump’s communications advisers or aides who provide him with a morale boost have been sitting at a remove. Natalie Harp, a former host on the right-wing OAN news network, who for years has carried a portable printer to supply Mr. Trump with a steady stream of uplifting articles or social media posts about him, is there. But she and others have been in the second row behind the defense table, or several rows back in the courtroom, unable to talk to Mr. Trump during the proceedings.

It is hard to recall any other time when Mr. Trump has had to sit and listen to insults without turning to social media or a news conference to punch back. And it is just as hard to recall any other time he has been forced to be bored for so long.

People close to him are anxious about how he will handle having so little to do as he sits there for weeks on end, with only a handful of days of testimony expected to be significant. It has been decades since he has had to spend so much time in the immediate vicinity of anyone who is not part of his family, his staff or his throng of admirers.

Over the next six weeks or so, Mr. Trump will have to endure more, including listening as prosecutors ask witnesses uncomfortable questions about his personal life in open court. On Tuesday, he’ll face a hearing over whether the judge agrees with prosecutors that he has repeatedly violated the order prohibiting him from publicly criticizing witnesses and others.

Most of the time, Mr. Trump has been forced to sit at the table, unable to use his cellphone, and listen as prosecutors have described him as a criminal, as jurors have been asked their opinions of him. Some of those opinions have been negative, with one potential juror made to read aloud her old social media posts blasting him as a sociopath and an egomaniac. The only times he has smiled have been when prospective jurors have referred to work of his that they have liked.

The highly telegraphed plan was for Mr. Trump to behave as a candidate in spite of the trial, using the entire event as a set piece in his claims of a weaponized judicial system.

But last week, in New York, Mr. Trump’s only political event was a stop at an Upper Manhattan bodega to emphasize crime rates in the borough. The appearance seemed to breathe life into him, but it also felt more like a stop a mayoral candidate would make than a presumptive presidential nominee. Some advisers are conscious of Mr. Trump appearing diminished, and they are pressing for more — and larger — events around the New York area.

Many in Mr. Trump’s broader orbit are pessimistic about the case ending in a hung jury or a mistrial, and they see an outright acquittal as virtually impossible. They are bracing for him to be convicted, not because they cede the legal grounds, but because they think jurors in overwhelmingly Democratic Manhattan will be against the polarizing former president.

But the shared sense among many of his advisers is that the process may damage him as much as a guilty verdict. The process, they believe, is its own punishment.