"fully losing" is a correct and usable phrase in written English.
You could use it to describe someone who has completely, utterly and totally lost something. For example: "Jack was fully losing his mind, trying to figure out where he had misplaced his keys."
He never fully lost control of the game, but lacked crispness over all.
The big salaries and flash offices have not fully lost their allure.
You never fully lose yourself in the simulation, if only because you're worried that it's impossible to look respectable while wearing a plastic face mask.
That universal souvenir printed with "My specified relative went to X location and all I got was this lousy T-shirt" has fully lost its irony.
Mario Andretti, who raced Formula One from 1968 to 1982, was the last American champion, but he was born in Trieste and never fully lost his Italian accent.
And, if the past fortnight is any indication, MasterChef has now fully lost the will to live and is content to simply go through the motions until someone takes it out back and shoots it.
After he cruelly provokes the suicide of the sweet-natured Sybil, Dorian is fully lost but shows none of it on his angelic face; the picture, meanwhile, ages and degrades and grows rotten in the attic.
In film noir, the hero is often a returning veteran, who, whether or not he has fully lost his identity, evidently has lost some important part of himself, and drifts through cities, anonymous and insomniac, in the attempt to locate it.
And yet many American Catholics never fully lost a fear that they could still be targets for the sort of cartoon drawn by Thomas Nast in the eighteen-fifties, which showed an apelike Irishman, whiskey bottle protruding from his back pocket, revolver at his side, shillelagh under an arm, slicing up the golden goose of the Democratic Party for the edification of a stark-eyed priest.
