Delays in stadium construction, infrastructure issues and protests have marred the lead-up to the FIFA World Cup, which starts on June 12.
Ronaldo, who is a member of the organising committee, said the nation's people had been let down.
"I'm ashamed for the Brazilian people who were expecting big investments and a great legacy for them, for us," he said.
"We promised to rebuild airports and improve transport and infrastructure. I am sorry for all the things we promised and have not completed.
"We have some numbers - only 30 percent of all the things that were promised will be done in time for the World Cup to get underway. That number is my only concern, for that I'm ashamed.
"The Brazilian people are the ones affected by our failure."
Ronaldo, the World Cup's all-time leading goalscorer and two-time winner, has big hopes for the national team.
The 37-year-old believes Brazil, who face Croatia, Mexico and Cameroon in Group A, come in with more confidence than the victorious 2002 team he was a part of.
"Brazil are entering the World Cup with much more confidence that we had when we entered the World Cup in 2002," Ronaldo said.
"I believe that the squad we have for 2014 is really strong. We have a balanced team, full of a lot of talent.
"Neymar is a fantastic kid, so talented, he is our great hope for the future.
"Another thing is that we are playing at home, this makes me really wish I were a few years younger and 45 pounds lighter. This is a once in a lifetime opportunity. Playing at home in front of our fans."
As for the goalscoring record he may give up to German Miroslav Klose, who has netted one fewer World Cup goal with 14, Ronaldo hoped he would remain top a little longer.
"I wouldn't say that I am not sleeping because of the chance of Klose breaking the record, but I am sending some bad towards him, perhaps just a little," Ronaldo said.
"My friends and fans are as well. However, whatever I do doesn't really matter, records are made to be broken by others.
"I don't have this ambition of being the biggest scorer forever, I don't think it is possible to do that."
