Louis Giglio has spoken out about his struggle with depression and his nervous breakdown while launching his new book.

The Comeback, published last week, focuses on stories of people coming through disappointment, pain, grief and depression. It is self-described as a book that "celebrates new beginnings," and "offers encouragement and perspective." Reviewers have written that it is positive and encouraging, that it carries a powerful message and is honest and transparent.

However, as well as telling the stories of others who have experienced depression and trauma, Giglio also reveals his own struggles with mental illness.

"I thought I was dying right there on the spot," he told The Blaze. One evening in 2008 he went to bed as usual, but woke up at 2am with a terrifying panic consuming him. He now describes it as a nervous breakdown.

"What followed was a very slippery and quick slope into a pit, which you can label anxiety, depression, fear, worry –all of it stemming out of depression," Giglio said. "And I spent about four months of my life in a tunnel of darkness. I thought I was going insane."

Although he says his mental illness has now improved, it remains a "thorn" which is always at risk of reappearing.

"I understand that at any day now, if I don't listen to God, I could end up in that place again," he said. "It's God's story of grace. It's not my story of strength."

The pastor of Passion City Church in Atlanta hoped his story and those of others he tells in The Comeback will help readers overcome their own battles.

Giglio is just one of 40 million adults in the USA who struggle with anxiety disorders. However, it is more common among pastors and one study estimated that 1 in 4 pastors will struggle with depression.

"When darkness falls, people tell their stories to their pastor, and so you walk through a lot of darkness with people," said Giglio.