Ruiz immediately surrendered his world title by losing to Joshua in the December 2019 fight, which came just six months after he rocked the sport of boxing with a knockout victory over AJ.
The fighter was criticised by many observers for failing to prepare adequately for the bout and Ruiz regrets his approach to the contest.
He said: "Losing [in] my last fight against Anthony Joshua, it was devastating, you know?
"Because I did not do the things that I was supposed to do. And I look back right now, and I’m like, ‘Damn, man, if I would’ve stayed dedicated, if I would’ve stayed disciplined and do the right things that I was supposed to do.’ And that was the most important fight of my life, [in] the rematch against Anthony Joshua."
Ruiz is set to make his return to the ring against Chris Arreola next month and believes that his vastly different approach to the fight will reap rewards.
He said: "But now, it’s totally different. Now I got my mind right. You know, I got a great trainer. So, we’re just ready for May 1st. But like I said, of course I was really disappointed because I knew the ability that I had to stay champion."
Ruiz felt that becoming a world champion took him a "little off the road" as he had achieved a life's ambition.
He told reporters: "What a lot of people don’t understand is that I’ve been waiting for this moment my whole life. And once that moment happened, you know, I kind of went a little off the road, you know? I started doing things that I thought I would never be able to do. I wasn’t really focused on the [Joshua rematch]."