Redknapp's side finished fourth in the Premier League at the end of the 2011-12 season, only to miss out on Europe's premier competition after the Blues lifted the Champions League trophy.
Redknapp, whose autobiography is being serialised in the Daily Mail, wrote: "You all know what happened. Chelsea were outplayed by Bayern Munich — as they had been by Barcelona and by Napoli in previous rounds — but they won the trophy.
"I was distraught. I felt like crying. It meant Spurs hadn't qualified for the following season's Champions League, despite finishing fourth — two points above Chelsea in the Premier League. They took our place as holders.
"I made one of the biggest mistakes of my life the night I went to watch Chelsea win the 2012 Champions League final."
Redknapp has been at QPR since November 2012.