The Wicklow-based actor shocked Hollywood back in June when he released a statement, saying he was finished with acting.
At the time, he said it was a "private decision" to pull the curtain down on his distinguished career.
But In an new interview with W Magazine, Day-Lewis said he was visited by a "great sadness" during the making of his last movie, which he's since been unable to shake off.
He said: "Before making the film, I didn’t know I was going to stop acting.
“I do know that Paul and I laughed a lot before we made the movie. And then we stopped laughing because we were both overwhelmed by a sense of sadness. That took us by surprise: We didn’t realise what we had given birth to. It was hard to live with. And still is.”
He added: “What has taken over in the past is an illusion of inevitability. But it’s settled on me, and it’s just there. Not wanting to see the film [Phantom Thread] is connected to the decision I’ve made to stop working as an actor.
"But it’s not why the sadness came to stay. That happened during the telling of the story, and I don’t really know why.
“I dread to use the overused word ‘artist,’ but there’s something of the responsibility of the artist that hung over me.
"I need to believe in the value of what I’m doing. The work can seem vital. Irresistible, even. And if an audience believes it, that should be good enough for me. But, lately, it isn’t.”
He also had this to say about his decision to release a statement about his retirement:
“I knew it was uncharacteristic to put out a statement, but I did want to draw a line. I didn’t want to get sucked back into another project," he stated.
"All my life, I’ve mouthed off about how I should stop acting, and I don’t know why it was different this time, but the impulse to quit took root in me, and that became a compulsion. It was something I had to do.
“Do I feel better? Not yet. I have great sadness. And that’s the right way to feel. How strange would it be if this was just a gleeful step into a brand-new life. I
"I’ve been interested in acting since I was 12 years old, and back then, everything other than the theatre – that box of light – was cast in shadow.
"When I began, it was a question of salvation. Now, I want to explore the world in a different way.”
There's now rumours that he might become a fashion designer, just like the character in his last film.
Addressing those rumours, Daniel Day-Lewis said: “Who knows? I won’t know which way to go for a while. But I’m not going to stay idle. I don’t fear the stony silence.”
The 60-year-old's last film appearance will be in director Thomas Anderson’s Phantom Thread, which hits the big screens in December.
The Oscar-winning actor's spokeswoman, Leslee Dart, confirmed the news to Variety magazine back in June. "Daniel Day-Lewis will no longer be working as an actor," Dart stated at the time.
"He is immensely grateful to all of his collaborators and audiences over the many years. This is a private decision and neither he nor his representatives will make any further comment on this subject."
Day-Lewis is the only male actor to ever scoop three best actor Oscars, winning for My Left Foot, There Will Be Blood and Lincoln. The British-Irish actor was also nominated for In the Name of the Father and Gangs of New York.