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After her husband's historic win in the 2008 presidential race, Michelle Obama wanted to stay put in Chicago with her girls and not move to the White House, according to "The Obamas," a new book by New York Times correspondent Jodi Kantor that is scheduled to hit stores on Tuesday.

The book describes Obama as "alone, frightened and unsure of what to do next" during her first days.

She worried about her children bumping into White House tourists during play dates. Later, she would acknowledge just how tough life at 1600 Pennsylvania Avenue can be.

"Sometimes it becomes difficult to live in what we call a bubble," she said, according to the book.

The world watched her on a trip to London in April 2009 when she visited with young girls - nothing out of the ordinary.

"We are counting on every single one of you to be the very best that you can be," Obama told them.

But what we didn't know, according to Kantor's book, was that Obama was having an epiphany, understanding for the first time through the eyes of those admiring girls what it meant to be first lady.

"She saw the responsibility, the impact, the potential of her role," Kantor writes.

In the early days, the pressure to both be perfect and look perfect was always on.

"Everyone was waiting for a black woman to make a mistake," an advisor told Kantor.

But Obama moved past that anxiety and took on a fight against childhood obesity and became a defender of her husband's drive to reform health care.

Obama examined what she wore, realizing that "everything she wore carried a meaning," Kantor writes. Her fashion became strategic, she wore glamorous ensembles at night and more relatable outfits bought at chain stores during the day.

Eventually, former White House aides say, Obama came to not only embrace, but love her role as first lady.

"It was natural that there would be a period of transition when she and the family went from being a private family in Chicago to the first family of the United States," former White House deputy communications director Jen Psaki told ABC News.