Speaking to The Observer, the X Factor judge revealed that he was "embarrassed" by his lack of success as a solo artist in the late 1990s.
Barlow recalled: "I was living in Cheshire at the time, going to London and back on the train.
"I used to heavily disguise myself, with a hat and everything. I was overweight, I wasn't feeling great. I was embarrassed to be me, to have people recognise me.
"It wasn't not being famous any more, or even not being a recording artist. It was having nobody who needed me, no phones ringing, nothing to do."
Referring to his songwriting frustration, the singer continued: "It was all a bit of a torment. I had this beautiful white piano, my lucky piano. Every hit I'd had I'd written on this piano. Within six months of this not happening any more, this piano drove me mad.
"To the point where I spent days just looking at it, lying underneath it, lying on top of it, rubbing my face on it, going slowly insane, trying to work out why this thing wasn't delivering to me like it used to."
The 40-year-old added that only when he rejoined Take That in 2006 did he feel "valid" again.
"That day was the first time in years that I got on the train, took my hat off and just sat there. People were coming up and being so nice. It felt so good," he said.
> Take That's Gary Barlow: 'Depression caused weight issues'
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Watch the video for Gary Barlow's flop 1999 single 'Stronger' below: