The Sons of Anarchy actor originally took on the role of Christian Grey in 2013 before later dropping out in October that year.
Speaking to Moviefone, Charlie added: "I wouldn't have signed up in the first place if I didn't want to do it. It was a really, really difficult time in my life. I was going to finish Sons at like 11pm Friday night, get on the plane Saturday morning to Vancouver for Fifty, missing the whole first week of rehearsal and start shooting Monday morning."
Charlie Hunnam in 2014
He continued. "And I was going to shoot that film, wrap that on the Wednesday and the following Monday I was going to start shooting Crimson Peak [Guillermo Del Toro’s 2015 film] in Toronto."
"I just had like... Frankly, something of a nervous breakdown. I just said, 'I can't... I can't.
WATCH JAMIE DORNAN BRING CHRISTIAN GREY TO LIFE IN FIRST FIFTY SHADES OF GREY TRAILER
"To fail on such a grand scale - because Fifty's going to be massive, it's going to be huge. I really didn't want to fail on such a grand scale and I just couldn't transition from Jax Teller (his Sons of Anarchy character) to Christian Grey in 48 hours. I bit off more than I could chew and it was painful. I loved the character and I wanted to do it.
Jamie Dornan and Dakota Johnson star in Fifty Shades of Grey
The role of Christian eventually went to British actor Jamie Dornan, who will star opposite Dakota Johnson in the film set to be released on Valentine's Day 2015.
In the first two-and-a-half-minute trailer, we see Christian and Anastasia (Dakota) meet for the first time when Anastasia, a shy literature student interviews the tortured billionaire.
The two begin a relationship - but Anastasia soon realises that Christian may not be telling her the whole truth about his private life.
Click here to watch the saucy Fifty Shades of Grey trailer - http://youtu.be/z4nJX8snP4s