The 38-year-old long-time rival of Australia's Anna Meares has previously spoken about her mental health issues.
But in an interview with the Daily Telegraph, she has given a graphic account of how matters deteriorated.
Pendleton, who retired from cycling after winning gold at the London 2012 Olympics, was diagnosed with severe depression after returning home from an aborted charity climb of Mount Everest.
"I had accumulated one-and-half times the dose of drugs to kill myself," she said.
"And I had it there, in front of me, and I knew how much it would take. And how long I would have to be left for it definitely to work. It wasn't even like I was really upset about it. I just felt numb.
"I felt horribly guilty for even considering it (killing herself).
"I said to my mum: 'Please would you forgive me (if I killed myself)?' Obviously, it was very upsetting for her to hear that.
"But I really wanted my family to be able to forgive me. Because... I wouldn't do it to hurt them on purpose. You just can't understand how much I was suffering on the inside."
After suffering a panic attack at a friend's house, Pendleton sought help.
She also turned to former British Cycling psychiatrist Steve Peters, who she telephoned during her darkest moment.
"I remember lying there with tears rolling down the side of my face. Not really crying, but just feeling a sense of hopelessness. I was so low. So helpless.
"And I just thought: 'I don't want to see tomorrow'. I'm so grateful that he (Peters) picked up (her phone call). Because I don't think I would be here if he hadn't."