"That culminated in winning an Oscar and that caused an epiphany which was, 'This isn't the answer'.
"I was holding a gold statue and I was the loneliest I'd ever been."
Interviewed on stage by Tina Brown, the WITW founder, Kidman also touched on her nine-year marriage with Keith Urban, a singer. She said: "We didn't really know each other - we got to know each other during our marriage."
Talking about her latest project, playing Rosalind Franklin, the DNA scientist, in Anna Ziegler's play Photograph 51, Kidman revealed she wept when she first read the script.
"I believe in Rosalind being celebrated and I wanted to put her back in the conversation and dialogue beyond the scientific community," she said. Now in its seventh year, the WITW conference aims to give a platform to global women "who are saving lives, enriching communities and shattering glass ceilings".
This year's line-up included Meryl Streep, the actress, Cara Delevingne, the model, and Leymah Gbowee, the Nobel Peace laureate, among others.
Delevingne, 23, talked about the difficulties of being a successful model. After kicking off her high heels, she said: "In our culture, we are told that if we are beautiful, if we are skinny, successful, famous, if we fit in, if everyone loves us - then we will be happy. But that's not entirely true. When you start out as a model you are constantly told you are not pretty enough, not tall enough, not skinny enough. You get battered and bruised a lot but then you have to grow a thick skin."
Also at the event was Theresa May, the Home Secretary, who said that women can be "clever and like clothes". Famous for her love of shoes, Mrs May said: "I think one of the challenges for women in politics and in business and working life is actually to be ourselves. You know what, you can be clever and like clothes. You can have a career and like clothes."
Nicola Sturgeon, the Scottish First Minister, touched on the subject of domestic violence. She said: "I believe more passionately than I believe in many things that we need to drive forward gender equality but we won't when so many women are still victims of violence," she said.
Speaking about the location for the conference, Ms Brown, 61, said: "Londoners have been leading the charge against inhumanity and inequality right from the days of the Suffragettes."