"After a while, you start to go on automatic pilot. There were too many times when I was sitting there bored," said Cowell during a Thursday appearance on The Oprah Winfrey Show.
"I thought, 'At the end of the day, the audience doesn't tune in to watch me being bored. They deserve more than that.' But I can't hide it when I'm bored. I just can't fake it."
Cowell won't have to fake it much longer, as next week's ninth-season American Idolfinale will bring an end to his to role on the Fox reality series' judging panel.
"You just know time's up. I remember the first year we did this, it was like an adventure. I loved that, not knowing if it was going to be successful. We were kind of making it up every week. I loved that buzz and I liked the adventure. It was fun," Cowell told Winfrey.
"Then you get to a point. For whatever reason, two years ago I remember sitting in the auditions one day and I thought, 'This is about as far as we can go.' I've always promised myself that I would leave before I was forced to go. It feels like the right thing to do."
It was only January when Cowell inked a deal with Fox to end his American Idol role and launch an American edition of his British The X Factor reality competition series for the network in Fall 2011, however Cowell said the network knew long before then that he was done with Idol.
"I'd warned them a year or two years ago, 'Look, once we come to the end of my contract, I'm going to go. So you've got to put some other people in place,'" Cowell told Winfrey.
WhileAmerican Idol's monster ratings have been in gradual decline for several seasons now, Cowell bluntly told Winfrey that he feels Abdul's departure does have something to do with this season's continued drop.
"I think we had a great team. She had her own reasons for doing what she did and I've stayed in contact with her a lot. But Paula, I was saying this about her the other day, she's a survivor. She's got great instincts. So for whatever reason, she decided not to go back," said Cowell.
"The show is a different show. But, it was going to change."
"I think it takes time for people to find their balance," he said.
"I think it was difficult for Ellen because the time was horrific. On her first day, I announced I'm leaving. She didn't know. I think that put her in a very tricky situation. Then everyone starts getting a bit paranoid about my replacement."
To say Cowell's American Idol replacement has been a hot topic this spring is an understatement, which he thinks is one of the problems with the show.
"I think it's too much about the judges at the moment and we've forgotten what the contestants are going through. I think primarily you've got to have someone on that panel who actually knows what they're talking about," he explained.
"If we were judging the ice skating Olympics, we wouldn't have somebody on there who's obnoxious. We'd have somebody who knows the difference between a 10, a 9 and an 8. I think to a point you've kind of got to go back to that."
As American Idol focuses on replacing Cowell, he has his sights set on launching The X Factor stateside -- even though he has no idea how it will be received.
"I'm excited about the new one. I have absolutely, genuinely no idea whether it's going to work or not. But I'll put it's success fate down to the fact: Can we or can we not find a star? I'm confident we can find a star," he told Winfrey.
"If you're going to do any show, you've got to have one belief, and that is you've got to find one star. Not 10, one. So we work over here because you've got the best singers in the world. So if I'm putting together a competition I have to believe there's somebody out there who I can turn into a star through the process."
Cowell also discussed some of the differences between the talent competition series he's leaving and the one he's launching.
"It's different because there's no upper age limit. I like the idea that somebody who could be 60-years-old can enter the show," he said about The X Factor.
"There's a lower age limit, we're taking it down to 14, and singing groups can enter the show. So when we created the show in the first place, we kind of sort of in our mind, it was like a David vs. Goliath potential final where a 16-year-old could be competing with a 35-year-old."
"Does Idol now become your competition?" asked Winfrey.
"Yes. But everything's our competition," responded Cowell, specifically citing ABC's Dancing with the Stars, which has managed to beat American Idol in the weekly viewership ratings several times this spring (Idol is still well ahead in the 2009-2010 season's overall viewership averages, averaging 25.2 million viewers to Dancing's 19.7 million).
"Whatever is popular is your competition. I've gone into this new show with just that one thought in my mind, which is if you make a great show and nobody likes it, there's nothing I can do about that. If you make a bad show it deserves to fail. That's the only way I can approach it. I'm taking anything for granted. I will work my butt of to make it as good as possible."
In addition, Cowell said he doesn't have the time to think about American Idol as competition.
"I have to not even think about that for a moment. What I have to do with the new show is to make the show I'd like to watch," he explained. "I promise you that's how I'm approaching it, which is all the things I'd like to see on a talent show. I'm going to try and make it happen."
While Cowell acknowledged that he's been at the center of a "lot of controversy" since American Idol premiered nearly eight years ago, he said the general public has also come to appreciate his insight.
"Within it all, I do trust my judgement as somebody who can spot a potential star," he said. "I do trust the fact that most people who I've met, they're very, very nice to me. What most people say is, 'You kind of say what I'm thinking."
One of the main reasons Cowell said he enjoys working on shows like American Idol and The X Factor is because it gives him an opportunity to spot those potential stars and help them succeed.
"If these people hadn't succeeded, what we did would have been a complete waste of time," he said. "It reminds me why we make these shows in the first place. Yeah, for ratings, for money -- everything is fantastic. But if you're not doing what you set out to do or actually promised, then you have failed."
The "best two examples" Cowell could think of as potential stars he helped unleash on America were third-season Idol winner Fantasia Barrino and fourth-season Idol winner Carrie Underwood.
"I don't know how I would define it looking back other than it's somebody that stands out in the crowd," he said, reflecting on those seasons.
"With Carrie, I can actually remember, it was almost as if the whole cast was in black and white. Only one person walked in in color. That's how clear it was to me that day she walked in. This girl knew what she wanted, luckily we spotted it."
Cowell added that what made Barrino a star was what she "had been through in her life."
"I made her the most likable contestant I've ever met," he said. "When you've got that massive likability thing going on, particularly in a competition like Idol, that carries you an awful long way. And she had this amazing ability to not be afraid of being emotional."
In addition to helping potential stars succeed, Cowell said he likes the buzz that a show like American Idol creates.
"You want to be part of something that people are talking about," he told Winfrey.
"I love that feeling. I genuinely, genuinely love that feeling. You feel that you're making people excited, they're enjoying it or they're not enjoying it -- whatever. I like that."
As his own American Idol journey comes to an end, Cowell said he has "more good memories than anything else."
"The final night will be quite emotional," he said. "I probably will be a bit emotional. But I've had a great time."