In their first appearance in a decider for 19 years, the Lions fell to Collingwood by four points in one of the greatest VFL/AFL grand finals in history at the MCG.
Joe Daniher's snap with 90 seconds left cut the Magpies' advantage from 10 points, but Collingwood, the close-game specialists, were able to hold on.
Fagan left a proud, but shattered Lions change rooms to front the media after the loss.
"We're well and truly in the window," the 62-year-old, who became the oldest grand final coach in VFL/AFL history, said.
"I think we've only just moved into the widow."
Fagan took over a broken club at the end of 2016.
The Lions endured two more seasons down the bottom of the ladder before shooting into finals in 2019 to end a 10-year drought playing in the post-season.
Brisbane exited the finals in straight sets in 2019 and 2021, and lost preliminary finals in 2020 and 2022, but those heartbreaks steeled them.
"Maybe because we've been in finals the last five years, the banter has been that maybe we'll miss our window," Fagan said.
"I think we're still moving into it. We've got to make sure we handle this loss well."
Fagan highlighted clubs being on the end of painful, close losses in grand finals being able to go one better soon after.
He believes Brisbane can join Geelong (2008 into 2009) Hawthorn (2012 into a 2013-15 three-peat) and West Coast (2005 into 2006) as teams that have bounced back from heartbreak in the best possible way.
"There's lots of teams in the history of the game that have lost close grand finals that have gone on to win premierships in the ensuing years," Fagan said.
"That will be my attitude, what can we learn from today to make us a better team next year.Â
"There's plenty of history around to say grand final losses don't have to define you or destroy you, they can make you. That's what we'll be looking to do."
That attitude seemed to have already rubbed off on the Lions players, with the rooms not as downcast as you would expect from a team that had lost in brutal fashion.
Fagan is adamant the Lions have most of the pieces already there to win the club's first premiership since 2003.
Father-son prodigy Will Ashcroft was on track to claim the Rising Star award before he ruptured his ACL in round 19.
Fellow son of a gun Jaspa Fletcher played an outstanding qualifying final and the teenager will only get better.
Ashcroft's brother Levi is due to be drafted to the Lions at the end of 2024.
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