The Portugal international, who is back playing for his country in a pair of Euro 2024 qualifiers, moved to Saudi club Al Nassr in January after becoming a pariah at United following an interview with Piers Morgan in which he was highly critical of the club and manager Erik ten Hag.
"There is no time for regrets in this life. Even if we don't do so well, it's part of our life," Ronaldo told a news conference before Thursday's match with Liechtenstein. "When we are at the top of the mountain, it's hard to see what's down here and many times I couldn't. I feel like I'm better prepared now because I can see some things. I'm a better man now.
"I think that everything in life happens for a reason. I'm often grateful to go through some difficult things so I can see who is really on my side. At the tough times, you see who is on your side. It wasn't a very good phase in my life, in my career, first on a personal level and then professionally."
Ronaldo is back playing international football for the first time since they lost to Morocco in the quarterfinals of the 2022 World Cup. In the interim Portugal hired Roberto Martinez to replace long-time coach Fernando Santos, who parted ways with the team after the tournament in Qatar.
"It was all in the balance after the World Cup," Ronaldo said of the possibility he would retire after last year's World Cup. "I reflected with my family and then we came to the conclusion that it was not time to throw in the towel.
"I learned a lot from it and I'm very happy to be back. The coach says he wants me in the squad. I feel that I still have a lot to give and I want to keep it going."
Ronaldo is set to break the all-time record of appearances with a national team after tying Bader Al-Mutawa's mark of 196 outings for Kuwait while playing in Qatar.
"Records are always positive," Ronaldo said. "They are my motivation. I like to break records and this record is special. I'll be really proud if it happens, but I want to keep playing even more games, I don't want to stop here."
Santos departure came amid the controversy of benching Ronaldo in the knockout stage of the tournament in Qatar, but the former Real Madrid star said it was time to move on from the past as they look to qualify for the European Championship in Germany next summer.
"It's a new chapter for everyone, for the players, the staff and the country," Ronaldo said. "We feel good energy. It's a breath of fresh air. I'm not saying that it's better or worse than before, but sometimes in life changes are positive. There are new ideas, a new mindset. You can notice it. You can feel the change and that is good for everyone."
Santos was in charge of Portugal for eight years, leading the national team to the Euro 2016 title -- the country's first major trophy -- and the 2019 Nations League title. He reached an agreement with the federation to leave two years before his contract was to end.
Martinez ended his six-year stint with Belgium after the team's disappointing group-stage elimination at the World Cup. He arrived pledging to respect Ronaldo's history with the national team and said Ronaldo will be among the team captains.
"I am surprised by the level of commitment of the players to the national team," Martinez said. "Everyone can play. We are starting a new cycle and we want the best. Ronaldo is part of the best and we need his experience."
In addition to Ronaldo, Martinez will inherit a squad filled with talented players such as Bernardo Silva, Bruno Fernandes, Diogo Jota, Goncalo Ramos and Joao Felix. The coach made few changes to the squad, with 40-year-old defender Pepe also returning to play alongside the 38-year-old Ronaldo.
"We were briefly introduced to him on the training field," Felix said about Martínez. "He explained to us some of his ideas, mainly for these two games, which are going to be very similar. He talked about the attacking dynamic, the connections between wingers and strikers."
Felix downplayed Portugal's chances in the matches against Liechtenstein and at Luxembourg on Sunday, games in which the 2016 Euro champions are considerable favourites.
"We always have to be prepared and not take any game lightly," Felix said. "We have the recent example of the World Cup, when everyone thought we were going to get past Morocco and we didn't.
"It's always complicated, it's always the best from each country, and everyone plays each game like it is a final. All games will be difficult, both at home and away. We have to be cautious."
Portugal are unbeaten against Liechtenstein, with six wins and a draw in seven matches.
Portugal's Euro 2024 qualifying group also includes Iceland, Slovakia and Bosnia-Herzegovina. The top two nations from each of the 10 groups qualify for the final.
Information from the Associated Press was used in this story.