The star began writing The Happy Prince, about Oscar Wilde in the years following his conviction for homosexuality to his death, 12 years ago.
It is finally being released in cinemas this week, and Everett, who has directed the movie as well as starring as Wilde, compared it to birthing a child.
“At the very beginning I had no notion that the whole process would take 12 years,” he said.
“It was like a long labour… agonising at times.
“It was always a question of taking one step forward and then two steps backwards, and it was demoralising, apart from the moments it went well.”
The actor, who became a household name after starring in My Best Friend’s Wedding with Julia Roberts in 1997, said when he first wrote the script he had no idea how long it would take to go into production.
After initially receiving a swift offer, Everett later refused the deal when it became clear one Hollywood producer wanted Philip Seymour Hoffman to play the role of Oscar Wilde instead of him.
“When I finished writing it Robert Fox, my producer, sent it to the greatest producer in America who’s a guy called Scott Rudin, who really makes the best films in the states.
“He rang back the next day and said ‘I love this film’ and I was literally at that point making acceptance speeches in the mirror.”
But it was not to be so straightforward.
“The day after that – and this is a very good picture of showbusiness, it’s like snakes and ladders – he rang back and said ‘by the way I don’t think you’re a very good actor and so I think Philip Seymour Hoffman should play Oscar Wilde’.”
Everett said his world came falling down.
“I was absolutely… that’s when I became bipolar, I was so upset, my world crashed, and I said no to him.”
Everett’s producer kept Rudin on board with the project, and they spent the next few years searching for a director, but after seven directors said no, he decided to do it himself.
“It’s been gruelling but it’s made me realise that the only thing really that counts is tenacity.”
Everett said he relates to Wilde, who he described as “one of the great punctuation points between the 19th and 20th century” as being at the “beginning of the gay movement”.
“For me as a gay actor trying to work in an aggressively heterosexual world, there are obviously parallels and obviously he is a source of historical inspiration,” he said.
Everett has previously spoken about struggling to win roles as an openly gay actor in Hollywood.
He believes the #MeToo movement will lead to change in the industry – and says he sympathises with how women have been treated.
“The thing is in our business, certainly, it’s a boy’s club, and I think the thing that has upset women about this boy’s club is that if you’re going to be a member of this boy’s club you have to kind of go with the rules of the boys, and I think really underneath it all that’s what the Me Too argument is about,” Everett said.
“A gay in the boy’s club has a very difficult position too because he’s really no more than a second-class citizen.”
He added that the industry is littered with double standards.
“At the moment it’s perfectly alright for a straight man to play a gay role, but it’s not perfectly alright for a gay man to play a straight role, and the boys club would say ‘it’s because you’re all such queens’ but actually a lot of straight men are quite queeny too.
“A lot of the gay actors are probably good enough actors to be able to play a straight role but they would say ‘oh no there will be no chemistry or this or that or the other’, but all these are lies, they are rules made up by the boys club,” he said.
“And I think that’s why I have sympathy with the Me Too movement, because it’s just this brick wall of traditional behaviour that is irritating actually.”
The Happy Prince, which also stars Colin Firth, Emily Watson and Colin Morgan, is out in cinemas nationwide on 22 June.