"When you take a lot of decisions, a lot, lot of decisions, you make mistakes," Guardiola, who will leave City after Sunday's final Premier League match at home to Aston Villa, told Sky Sports in an interview.
“There is one regret that I have deep inside for many years, that I didn’t give a chance to Joe Hart to be with me to prove himself how good a keeper he was. I could have said, 'Okay, Joe, let’s try to do it together. If it doesn’t work, okay, we’ll change it.' But it happened.
"I have to take decisions and sometimes I’m not fair enough. Maybe with time then and learning... but I regret it from that time."
After taking over in 2016, Guardiola sent England custodian Hart on loan to Torino and West Ham. Hart permanently left for Burnley in 2018, after 348 appearances for City over 12 years.
Later, he briefly played for Tottenham before concluding his career at Celtic.
Guardiola, who led City to six Premier League titles, a Champions League triumph and the Club World Cup, said that dealing with human emotions was the toughest part of being a manager.
"If in that I failed, I do apologise, but it never, never was my intention or the intention of the club," the 55-year-old Spaniard added.