Adams, MP for Selwyn, said her announcement may have come as a "bolt from the blue" for others, but she had been thinking about it for some time.
She said her decision was a difficult one.
"The day I don't think I can give it all of the energy and hunger and wake up in the morning just wanting to rip into it, that's the time to go.
"I want my life back ... I want to spend more time with my family."
The 48-year-old said the decision had nothing to do with losing the leadership contest to Bridges last year, and she thought the party could win the 2020 election.
It would be an "amazing privilege" to be the Finance Minister if National won in 2020, but she didn't want to stay to find out.
"These jobs need 100 per cent of your energy and passion, and while the heart is there, I just don't know if the energy is."
She said she had made the decision a couple of months ago and had told Bridges some time ago.
But she only broke the news to caucus this morning, and the reaction had been "surprise".
"Putting it all out there is a big call," she said, adding that she had become emotional at the caucus meeting.
She also fought back tears as she spoke of the staff that she had worked with and the people of Selwyn that she represents.
Asked about her hardest day in politics, she said: "It was early on and my daughter was at boarding school ... she rang me up, late at night, she was at boarding school, she wasn't getting on with her friends, and I felt a million miles from her."
Her family were "embarrassingly excited" about spending more time with her.
She didn't believe in "a life in politics", and she didn't want to risk becoming bitter and half-hearted by sticking around.
Adams said she would give up her shadow portfolios, but continue to work hard for her Selwyn constituents in the time she had left in Parliament.
She made the announcement today to give the next finance spokesperson for National enough time to get into the role before the 2020 election.
National's spokesman for economic development Paul Goldsmith would be a front-runner to take over the role.
Adams was considered a strong minister under the leadership of English and John Key, holding a variety of portfolios at various times including justice, communications, broadcasting, courts, internal affairs and the environment.
Her proudest moments in those roles included implementing national environment reporting standards, the rollout of fibre and broadband, and expunging homosexual convictions.
She had wanted to continue to tackle family and sexual violence, and was glad the Government had continued that work.
She had no idea what she would do next, and that was "exciting".
"I honestly just haven't thought about it."
Adams was also Associate Minister for Canterbury Earthquake Recovery, and has previously spoken about the how the 2010 earthquake shook her home with such force that it threw a fish tank across her daughter's bedroom, shattering on the bed inches from her head.
Adams is a former lawyer and mother of two. She lives on a farm in Aylesbury, Canterbury.
She was raised by her mother Lyn after her father left when she was 2.
When Adams was 11 she shifted to live with her father in Auckland for a few years, attending Rangitoto College before going to Canterbury University.
She voted for Act when Don Brash was leader of National in 2005, campaigning on one law for all and hardline welfare reforms.
Three years later Adams quit her post as a partner in commercial law and entered Parliament as a National Party MP.
She lost her mother to melanoma and supports euthanasia - with the right controls.
Scott, who also announced his resignation today, said: "I have every confidence that National will claim victory at the next election."
Scott has been the MP for Wairarapa since 2014.
