Phelps' lack of interest led to a blow-out fight with his longtime coach Bob Bowman and he didn't show up for practice for 10 days as the start of the London Olympics loomed.
Despite his lack of enthusiasm, Phelps added six more medals to his mantle, including four gold, but as his professional career appeared to be going from strength to strength, his personal life was spinning out of control, resulting in an arrest for driving under the influence in 2014.
"I sent myself down a downward spiral," he continued.
"I think it was a sign more than anything else, that I had to get something under control, whatever it was. I look back at that night and everything happened for a reason."
Phelps believes his run-in with the law - his second alcohol-related arrest in 10 years - was a cry for help: "Honestly, at one point, I felt like I didn't want to see another day," he added.
"I felt like it should be over."
He subsequently checked himself into Arizona's The Meadows rehabilitation centre for 45 days of treatment, but despite his penchant for booze, Phelps does not consider himself to be an alcoholic.
"I don't know (if I am an alcoholic)," he said. "I know I have moments where I have gone off the deep end where I shouldn't, and I would say (I've) binged more than anything else."
Asked if he believes he has a drinking problem, he defiantly stated, "No, because you can put a beer in front of me, or an alcoholic drink in front of me, and I won't feel the urge to drink it."
Now, Phelps insists he is a changed man and he is focused on success at the 2016 Summer Games in Rio de Janeiro, Brazil - his fifth and final Olympic Games.
"I want to be here," he said. "That's the difference. I had no desire to go work out before. I wanna retire how I wanna retire and I have a great opportunity to do that. I haven't trained like this in... a decade."
Phelps has also turned his personal life around and is set welcome his first child with fiancee Nicole Johnson.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
If it is an emergency and you feel like you or someone else is at risk, call 111.
• Canterbury Support Line: 0800 777 846
- WENN