Ms Davidson said the thought of being away from her family to campaign had started to "fill me with dread".
Reports had emerged on Wednesday that she would stand down, and Ms Davidson confirmed she is quitting on Thursday.
Ms Davidson said the main reason for her resignation was the birth of her baby boy, Finn, in October last year.
She wrote in her resignation letter: "I fear that having tried to be a good leader over the years, I have proved a poor daughter, sister, partner and friend.
— Ruth Davidson (@RuthDavidsonMSP)
"The party and my work has always come first, often at the expense of commitments to loved ones. The arrival of my son means I now make a different choice."
Ms Davidson, who had led the party for eight years, admitted the "conflict" she had felt over the Tories' Brexit approach amid clashes with Prime Minister Boris Johnson.
But she reiterated: "The biggest change... has been starting a family."
In an Edinburgh news conference shortly after tweeting her resignation letter, Ms Davidson added: "I have to be honest... where the idea of getting on the road to fight elections would once have fired me up, the threat of spending hundreds of hours away from my home and family now fills me with dread.
"And that is no way to lead."
Though Ms Davidson mainly focused on the personal reasons for her resignation, she also blasted hardline Tory Brexiteers in Westminster for not passing Theresa May's Brexit withdrawal agreement, which was defeated three times earlier this year.
"I think we had three golden opportunities to support a deal," she said.
"I think the people right now who say they will do anything to avoid no-deal had a goal gaping in front of them three times and they hit it over the bar."
She also had choice words for opposition leaders including Jeremy Corbyn and Jo Swinson following their summit this week on how to avoid a no-deal Brexit.
"For all the elaborate plans of bringing down governments and installing Ken Clarke and Harriet Harman [as caretaker Prime Minister], the simplest way to avoid no-deal is to vote for a deal."
Ms Davidson also said she would be quitting the Scottish Parliament in 2021.