The singer, who releases new single 'Candy' on October 29, explained that he tired of his career after the moderate success of Reality Killed the Video Star, Rudebox and Intensive Care.
"I feel like my navigation system was broken back then and like I wandered around aimlessly," he said, according to STV.
"It was liberating anyway. However, I didn't really care about my career in 2006. I just felt incredibly lethargic and like I couldn't live up to the requirements of being a popstar."
Asked if he wanted to quit, he added: "Quite definitely. Mentally I had already waved my career goodbye.
"I had started to paint and wanted to become a serious artist. Unfortunately I'm an awful painter. And I'm not very good at making radical decisions. Otherwise I would have called it a day."
Williams will reportedly join the X Factor panel for one of the live shows later in the series.
Watch the video for 'Candy' below: