getty (2) Dominic Monaghan and Evangeline Lily
"I think I've only really got my heart broken once in my life," Monaghan recalled. "I was dating an actress on Lost called Evangeline Lilly, who, you know, has gone on to have a name all of her own. And yeah, you know, I don't really know how she would explain the narrative, but from my point of view, it was probably the first time in my life that I was just kind of all in."
Despite his commitment, Monaghan admitted he had "an extraordinary amount of growing to do in terms of handling my substances and handling my choices."
He noted that he was raised in England, where it's not unusual for someone to begin drinking alcohol as a teenager.
"And that's not like a glass of wine or a pint of beer. That's like getting absolutely wankered on a Friday and Saturday night and maybe a Sunday afternoon," he explained. "I thought that was normal. And I had a huge amount of growing to do in terms of what was normal and what wasn't normal."
Vivien Killilea/Getty Dominic Monaghan
RELATED: Where Are They Now? The Cast of Lost
As for how he and Lilly, 43, parted ways, Monaghan claimed he found out they were no longer together after she began dating someone else.
"I mean, to say I was devastated is an understatement, you know. I had to be told by a third party like, 'You do realize that she's with this guy,' when we were together. And I was like, 'Wait, what?; And this person was like, 'Yeah,'" he recalled.
Despite his heartbreak, Monaghan said he's grateful for the relationship he shared with Lilly.
"I've broken up with girls before, and when I've looked at it I've thought, 'Well, she's a lovely person, but it was never gonna work,'" he explained. "But certainly, when I was with Evie, I was like, 'This is fine. The little things that, you know, we're working on, we will figure out. And we will make beautiful shapes with each other into eternity.' And I think like, losing that potential was super sad. But it also got me to where I am as a person. So I'm very happy that it all happened."
RELATED VIDEO: Dominic Monaghan Shares His Fondest Memory from the Set of Lost
Lilly portrayed Kate Austen for all six seasons of Lost, while Monaghan portrayed Charlie Pace for all but the fifth season. Despite their longtime roles, the former pair previously confessed that they rarely — if ever — revisit episodes of the ABC sci-fi drama, which wrapped in 2010.
"I'm aware that it was a good show and I'm aware that clearly people are continuing to like it now," Monaghan told PEOPLE earlier this month. "People will come over to me on the street and they've written, 'Not Penny's Boat' on their hand or people will tell me that it was their favorite character or that their kids have just got into it."
"It's certainly found an audience and continues to do so," he added. "It became kind of a piece of pop culture. I was happy to be involved in a show that for a while there was the biggest TV show in the world."
Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.
As for Lilly, she told PEOPLE last October, "I cringe watching every single episode of the first 2 seasons when they were airing back in 2005. Because it was my first acting gig ever, and I really didn't know what I was doing, and I was so bad in so many moments. I was so cringe-worthy."
Still, she's not completely against having a binge-watching session one day.
"I think I will definitely at some point sit down and do a Lost marathon, and I think hopefully I imagine there will have been enough space that it will just be the most beautiful walk down memory lane and really put a big smile on my face remembering all the people and all the places and you know all the memories that were made on that show," she added. "It was a very intense time for all of us, and the memories are — they mean a lot to me at this point. It's become such a beloved treasure in my life."