The 41-year-old Texan told chat show host Oprah Winfrey he had used the blood-boosting agent EPO, the human growth hormone, testosterone, cortisone and blood doping.
Armstrong, who last October was stripped of all his Tour titles, said that at the time of his drug-taking he did not feel it was wrong.
He said he did not feel bad about taking performance-enhancing drugs, nor did he feel it was cheating, as he was creating a level playing field with other riders who took drugs.
He said: "I looked up the definition of a cheat: to gain an advantage. I didn't view it that way. I viewed it as a level playing field."
But he said he had now changed his opinion, telling her: "I'll spend the rest of my life trying to earn back trust, apologise to people.
"I see the anger in people. And betrayal. It's all there. These are people that supported me, believed in me. They have every right to feel betrayed. And it's my fault.
"I made my decisions. They are my mistake. I acknowledge that and I'm sorry. I deserve this."
Armstrong told Winfrey he felt doping was necessary to win the Tour de France.
He said: "It was part of the job. I made those decisions."
Armstrong, who was also banned from sport for life, denied doping during his comeback 2009 and 2010.
He also said he wished he had co-operated with the United States Anti-Doping Agency investigation, which proved his downfall.
"This story was so perfect for so long," Armstrong, who confirmed his doping in a series of answers to yes-no questions, told Winfrey.
"I try to take myself out of this situation and look at it: you overcome the disease, you win the Tour de France seven times, you have a happy marriage, you have children.
"It's this mythic, perfect story and it wasn't true."
The myth of the cancer survivor turned serial winner, which Armstrong perpetuated, captivated millions.
Asked if it was hard to live up to that image, Armstrong said: "Impossible. The story is so bad and so toxic, a lot of it's true."
Armstrong was asked why he chose to confess his misdemeanours now.
"I don't know that I have a great answer," he said. "This is too late. It's too late for probably most people and that's my fault.
"I view this situation as one big lie, that I repeated a lot of times. It wasn't as if I just said no and moved on."
Armstrong insisted it was his successful battle with testicular cancer which increased his desire to win at all costs.
He said his decisions were based on a "ruthless desire to win" and did not feel wrong to him at the time.
Armstrong bypassed the testers by clever "scheduling".
"I didn't fail a test," he said.
Armstrong insists he is willing to co-operate with the authorities in future.
The second part of the interview will be broadcast tomorrow.