Zayn Malik has said he suffered with anxiety and an eating disorder during his time with One Direction.
The 23-year old pop star is set to release his autobiography Zayn, in which he talks of a time when he was so busy, he would forget to eat.
"When I look back at the images of myself from around November 2014, before the final tour, I can see how ill I was," the former One Direction star writes on his book.
Malik has been public about his anxiety since he left the boy band in 2015, saying he was "stressed" and just wanted "to feel like a normal 22-year old."
In an interview before Tuesday's book launch, the singer said that during his tours with One Direction he was "caught up with other things, like going out, drinking or partying".
"I realised that I wasn't eating as much just down to the amount of work that we were doing ... our schedule was kind of crazy so we were all over the place," he said.
"A bit older and a bit more wiser," Malik now says he is now better at managing his eating, and says he is "massively grateful for the opportunity to be in that group".
Earlier this year, he said in an interview he "never really wanted to be part of the group," but "gave it a go, because it was there at the time".
He left the group because he "felt lost", and "wanted to write his own lyrics".
"What you've got to understand is that none of us really had much say in the music," he writes in his book.
Malik's solo album, Mind of Mine, debuted at No. 1, but the star cancelled performances due to anxiety.
In June, he bowed out of his appearance at the Capital Summertime Ball perform in London at the Wembley Arena, where he previously performed with 1D.
"I felt sick. I couldn't breathe," he writes about the morning of the show. "The idea totally freaked me out and I was paralyzed with anxiety."
The Pillowtalk star says he is now happy with his career, without disclosing many details of his personal life or his relationship with American fashion model Gigi Hadid.
The couple have been together since 2015, and Hadid has been public about her support for Malik's struggles.
Earlier this year, she wrote on her Twitter she was "so proud" of him and his "bravery and honesty".
When asked about his time with One Direction, Malik said he felt the band was a "juggernaut" he could not control.
When asked if he's still friends with his former band mates, he said: "Yeah we're talking - some of us are."