Barry, who rode for the U.S. Postal Service team from 2002 to 2006, was among 11 former team-mates of Armstrong's to provide sworn testimony for a United States Anti-Doping Agency (USADA) report investigating the seven-time Tour de France winner.
"As a boy my dream was to become a professional cyclist who raced at the highest level in Europe," Barry said in a statement on his website. "I achieved my goal when I first signed a contract with the United States Postal Service Cycling team in 2002.
"Soon after I realised reality was not what I had dreamed. Doping had become an epidemic problem in professional cycling."
Barry said that shortly after joining the U.S. Postal squad he was pressured into using performance-enhancing drugs, becoming part of what USADA called "the most sophisticated doping program in sport."
"After being encouraged by the team, pressured to perform and pushed to my physical limits I crossed a line I promised myself and others I would not: I doped," said Barry, who represented Canada in the 2008 Beijing Olympics. "It was a decision I deeply regret.
"It caused me sleepless nights, took the fun out of cycling and racing, and tainted the success I achieved at the time. This was not how I wanted to live or race."
In 2006, the year after Armstrong claimed his seventh Tour de France title, Barry said he quit using performance-enhancing drugs and became an anti-doping crusader, although until Wednesday's report was released he had never admitted to doping.
After being contacted by USADA, Barry said he decided to come clean to help improve the sport for young cyclists.
"From 2006 until the end of my career in 2012, I chose to race for teams that took a strong stance against doping," Barry said. "Although I never confessed to my past, I wrote and spoke about the need for change.
"Cycling is now a cleaner sport, many teams have adopted anti-doping policies and most importantly I know a clean rider can now win at the highest level.
"I apologise to those I deceived. I will accept my suspension and any other consequences. I will work hard to regain people's trust." (Reporting by Steve Keating in Toronto; Editing by Eric Beech)