Max George was "mentally breaking down" at the height of his fame.
The former Wanted singer has opened up about his struggles with depression, admitting he experienced his first "proper episode" when the group's debut single, 'All Time Low', hit the top spot in the UK singles chart in 2010.
He said: "It doesn't matter what you do or who you are, you can literally have everything you want, but as success happened, mentally I was breaking down.
"My first proper episode was when The Wanted went to Number One. I'd achieved everything I had ever wanted but I couldn't handle how it exploded so fast."
The 32-year-old singer took a long time to "accept" his struggles because he couldn't understand why he'd been gripped by the mood disorder.
He told the new issue of Britain's HELLO! magazine: "It is a question I have been asked before, if I'd had any problems in my life with mental illness and, to be honest, I have not told the truth. I always would say no.
"It took me a hell of a long time to accept myself that I had these problems, and then to be able to speak to my family and close friends because it was so out of character for me."
The 'Glad You Came' hitmaker was diagnosed with depression while working on 'Glee' in Los Angeles after undergoing numerous tests to identify the cause of his physical symptoms.
He said: "I know it is a mental illness, but it was a physical illness for me, too. From getting out of bed in the morning, I'd get in the lift to go down from my apartment, and I'd get this overwhelming feeling, a bit like claustrophobia, and I'd have to run out and be sick, I couldn't handle anything.
"I had brain scans and every test under the sun, and they kept saying there is nothing we can physically see, but I knew there was something badly wrong."
While Max's partner, Stacey Giggs, has been very supportive in their two years together, he didn't open up to her about his struggles but she was by his side during "an episode".
He recalled: "I remember I'd just had a really bad day with it, and she said, 'What is wrong with you? What do I need to do,' and I said: 'Just try to be normal around me. There is absolutely nothing you can do, unless I'm actually curled up in a ball on the floor and need you to pick me up'."