The former cyclist said his attempts to destroy team masseuse Emma O'Reilly's reputation were "inexcusable and embarrassing".
Armstrong has walked to the border of truth many times before but has yet to cross it; now he smells a chance at a reduced ban from sport.
Sir Bradley Wiggins said it was "emotional" watching Lance Armstrong admit to using performance-enhancing drugs.
Lance Armstrong was close to tears when he spoke about his having to tell his son not to defend him.
He said: "When this all really started I saw my son defending me and saying 'what you are saying about my dad is not true'. And almost goes to this question of why now.
"That's when I knew I had to tell him. And he had never asked me, he had never said 'dad is this true?' He trusted me.
"At that time I didn't say something, but I knew I had to."
– Lance Armstrong
Frankly - this may not be the most popular answer - but I think I deserve it [to be able to compete again], maybe not right now. But if you look at the situation, you look at the culture, and the sport, and you see the punishments. Okay you are trading my story for a sixth month suspension? That's what everybody got. So I got a death penalty (can never compete again). I am not saying that is unfair necessarily, but I am saying it is different.
When asked about whether he was doing the interview with Oprah so that he would be able to race again, Lance Armstrong replied 'hell yes'.
– Lance Armstrong
Do I want to compete again. The answer is 'hell yes. I am a competitor. It's what I have done my whole life. I love to train, I love to race, I love to tow the line. I don't expect it to happen. Not the Tour de France, but there are a lot of other things that I could do, but I can't. With this penalty, with this punishment - and I made my bed - but if there was ever window, would I like to run the Chicago marathon when I am 50? I would love to do that, and I can't.
Lance Armstrong has said that severing all ties with his Livestrong Foundation was his most humbling moment.
– Lance Armstrong
That was the most humbling moment. To get that call - two parts. One: step as chairman, but stay on the board - stay involved. [But] that wasn't enough for the people, for our supporters. Then a couple of weeks later the next call came - 'we need you to step aside'. It was the best thing for the organisation, but it hurt like hell. That was the lowest.
In the second interview with Oprah, Lance Armstrong admitted he felt disgraced and humbled.
He said: "But I also feel humbled. I feel ashamed. This is ugly stuff.
"Nike called, and they said that they are out. Then the calls started coming.
"Everybody was out. Still not the most humbling time, but not a fun period."
The second part of the Oprah Winfrey interview is set to air at 2am GMT, with further revelations expected on how his family have coped while the cyclist was challenged over doping allegations. Last night he admitted he had taken banned substances during all seven of his Tour de France wins.
Oprah Winfrey's exclusive interview with disgraced cyclist Lance Armstrong was watched by some 3.2 million Americans, the second highest audience for her struggling channel OWN, the U.S cable channel said.
OWN said the most-watched telecast in its history was Winfrey's interview with the family of the late Whitney Houston in March 2012 that drew 3.5 million viewers.
Chat show queen Winfrey quit her daytime TV show in 2011 to launch the Oprah Winfrey Network (OWN), a joint venture with Discovery Communications.
The World Anti-Doping Agency (WADA) has urged Lance Armstrong to make a "full confession under oath" following his doping admission to Oprah Winfrey.
The WADA said it was "interested" to listen to his confession, but reiterated that it has made "no difference" to his status as a life-time banned athlete under the World Anti-Doping Code.
– A statement from the World Anti-Doping Agency
If Mr. Armstrong truly wants to make amends for his doping past, then he needs to make a full confession under oath to the relevant anti-doping authorities.
He must make a sworn statement that reveals the full truth and contains information that will assist the fight against doping in sport.
Tyler Hamilton, a former team-mate of Lance Armstrong who implicated him in the doping scandal, has told ITV News we're likely to see a "new Lance Armstrong" now he has confessed.
Speaking about his own experience, Hamilton said in his only UK interview, "I was a broken man two-and-a-half, three years ago, and I finally did the right thing ... telling the truth felt great. It sets you free".
The second part of Lance Armstrong's interview on the Oprah Winfrey Network will be shown in the US tonight.
During the interview, Armstrong discusses the impact on his family, the companies who sponsored him, his charity Livestrong and what he hopes the future holds.
UK viewers can watch it online on Oprah.com at 2:00am tomorrow.