Mark Webber says he had struggled with motivation in recent years before deciding to leave Formula 1.
The Red Bull driver, 37, is to lead Porsche's new assault on endurance racing after 12 years in F1.
"I've been on the edge with F1, motivation-wise, for the past couple of years," he told F1 Racing magazine.
"You have to be driven. You turn yourself around each winter and the fire in the belly is not quite what it was when you were 24."
Webber, who has won nine grands prix and narrowly missed out on the championship in 2010, thinks he may be leaving F1 "a year too soon".
"I still enjoy being belted in the car and driving down the pit lane, which is a bit disappointing, but I've spoken to some really good sportsmen and women who have been at the crossroads where making the call was not easy and they messed it up," he said.
"I'm probably leaving F1 a year too soon but with the [regulation] changes next season [the introduction of turbo engines with a fuel limit] and the opportunity to join Porsche, it's the best move for me."
The Australian had got to the point where he was questioning whether he wanted to race in F1 any more and is sure he made the right decision for his future.
"I remember hearing something about sportsmen and women years ago," he told the latest issue of the magazine in an exclusive interview. "They were saying that as long as they could keep their motivation, they would keep going.
"I could never work out what that meant. How could you lose your motivation? But questions keep coming to me more and more often that were never there in the past.
"It's not about driving or racing, it's about keeping my own F1 programme going for 11 months of the year. And it's just got to the point where it's like, well, I've achieved a lot of things."
Webber admitted certain aspects of being an F1 driver had begun to pale.
"Travel and hotels," he said. "And probably the repetitive nature of the job. A bit of media. Lots and lots of small things that you're happy to deal with when you're bright-eyed and bushy-tailed.
"But it does, in the end, force you to ask yourself the question: 'Do I have to be here, doing this?' And when Porsche came along, I could look myself in the eye and say: 'Well, you know what, I probably don't have to do some of those things any more."
Red Bull have named fellow Australian Daniel Ricciardo, 24 as Webber's replacement.