But with the Rio Games five months away, the record-setting Olympian believes he's on track to change that, he said Tuesday during an appearance at Under Armour's Baltimore headquarters.
Phelps has trained in Arizona since last summer but was back in his hometown to unveil a new Under Armour commercial and to discuss his Olympic preparations with a crowd of international journalists.
"You guys will see a difference and a change," he said. " I'm more relaxed in my own skin than I've ever been. I'm the happiest I've ever been. And it's showing in my performance."
Phelps made light of what a busy year he's facing. Not only is he training for a fifth, and likely final, Olympics, he and fiancee Nicole Johnson are expecting their first child in May and planning their wedding.
"We just decided to do it all in one year," he said. "Why not?"
Through it all, he's held to a rigorous training schedule that puts his preparation for the 2012 Olympics to shame. Phelps said he hardly cared about swimming in the run-up to the London Games, where he still managed to win four gold medals.
But these days, he's always in bed by 10 p.m. and has voluntarily added Sunday workouts to his regimen. Though he was traveling for media obligations the last two days, he still found his way to a pool each morning, He's down from 13-percent body fat four years ago to 5 percent now.
Phelps' devotion to swimming is emphasized in the commercial, which intersperses images of his daily rituals with a long shot of him cutting through the water in a darkened pool, set to a song called "The Last Goodbye" by the Kills.
On Monday, Under Armour posted footage of Phelps and Johnson tearing up when they saw a rough copy of the spot during a Christmas visit to Baltimore.
"It really just shows who I am," he said. "And that's what I wanted."
After he discussed the commercial, he went through a public workout at Under Armour's on-campus gym with longtime trainer Keenan Robinson, hefting bulky weights and doing pull-ups as he swung along a succession of metal bars hanging from the ceiling.
Johnson and Phelps' mother, Debbie, looked on.
Afterward, Phelps said he's spent an unusual amount of time in the last year looking at his gold medals, remembering what he thought and felt as he received each one.
"The journey has a couple more months left," he said.