"I'd give myself a red card for last week," McIlroy joked, citing Manchester United's loss a day earlier, as he opened his press conference Wednesday at Trump Doral.
When the topic turned to McIlroy's decision to walk off the golf course mid-round five days ago at The Honda Classic, he was apologetic.
"It was a mistake," McIlroy said. "Everyone makes mistakes. I'm learning from them. Some people have the pleasure of making their mistakes in private. Most of my mistakes are in the public eye."
McIlroy issued a statement after withdrawing last Friday that said he was suffering from a toothache. He reiterated Wednesday the statement was true, but he also said his mental state was not good.
"I wasn't in a good place with my golf game, my head was all over the place," McIlroy said. "I realized pretty quickly that it wasn't the right thing to do. No matter how bad I was playing, I should have stayed out there.
"There's no excuse for quitting. I let a lot of people down and for that I'm sorry."
McIlroy was open and honest throughout his press conference, which played to a packed interview room at the Trump Doral Resort and Spa and lasted a little over a half hour.
The 23-year-old World No. 1 also addressed was his switch to Nike, saying his swing is the problem, not his equipment.
McIlroy spent the weekend working on the latter at The Bear's Club under the watchful eye of his instructor Michael Bannon.
"I know it's purely the swing," McIlroy said. "The equipment is fantastic. When I make a good swing it does exactly what I want."
He also addressed rumours that he and tennis playing girlfriend Caroline Wozniacki had broken up, which some had speculated was at the root of his problems.
"Just because I have a bad day on the golf course, and Caroline loses a match, does not mean we are breaking up," he said.
McIlroy will tee off at 11:53 a.m. ET on Thursday with Tiger Woods and Luke Donald.
"I've got to remember I started to play golf because I love it, I really do," McIlroy said. "I've got four rounds, thankfully. I haven't been enjoying it because I've been putting so much pressure on myself. I have to go out there and enjoy myself."