Speaking about her writings, Michelle said: "It has allowed me, for the very first time, the space to honestly reflect on the unexpected trajectory of my life".
She said sometimes young people enter in to marriage too hastily. I have been at probably every powerful table that you can think of, I have worked at non-profits, I have been at foundations, I have worked in corporations, served on corporate boards, I have been at G-summits, I have sat in at the United Nations, they are not that smart.
Obama is due to speak to an event at London's Royal Festival Hall on Monday evening. Facebook has since put out a statement saying that Sandberg "takes full responsibility for any activity that happened on her watch".
She said that the idea that women can have it all at the same time is "a lie".
Obama's blunt assessment came as she was discussing how women can achieve the balance in life that allows for a successful career and marriage, NY magazine reported.
"And I just find it really inspiring". I was getting real comfortable up in here.
Her husband Prince Harry, 34, enjoys a close relationship with Mrs Obama, who has been a firm supporter of his Invictus Games. All kids can only dream things that are known to them.
"We can't wait until we have two years out from college".
Asked if she still believes in her motto "When they go low, we go high", she said: 'My answer is, "What choice do we have?"'
But she also had a serious message about the need for girls to get a good education and to overcome self-doubt - particularly young black girls.