Ronaldo is back for today's match at Everton and he said: "I have taken a lot from this punishment.
"I am mentally prepared now to tolerate anything that my rivals can throw at me. This is a personal promise that I have made to the manager. It will never happen again and that has to be a positive thing for Manchester United and myself."
Ronaldo has been restricted to the frustrated role of spectator during his three-game ban, describing it as the most frustrating period of his professional career.
"I would not wish the last few weeks on anybody," he said. "I am a person who lives for football and over the last month I have suffered more than at any time in the last three years. It has been an incredible sensation not to be able to help the team and it will not happen again."
United have struggled in Ronaldo's absence and he admitted it had been an indifferent start to the season.
"We have not seen the real Manchester United so far," he said. "This is already different to the last campaign but we still have time to win the title again. During my absence the team has won two matches and now we are in better form. If we analyse the table, the points difference is not that great."