Disclaimer: Not mine, folks.
NOTES: Inspired by Larna Mandrea's wonderful oneshot, Desperation. It's much better than this, so I encourage you to go and read it.
It's terribly easy, in the end, to decide to lose himself in his reflection. Of course he remembers what Dumbledore told him so many years ago - that it doesn't do to dwell on dreams - but the memory of that encounter itself seems a dream, and there's only the here and the now to occupy his mind.
And the here and the now is in this room, in front of this mirror, for the rest of eternity.
After he defeated Voldemort, he started to think, to remember. The first time he looked in the mirror, he saw his family, but that wish never came to fruition. Then he saw himself with the stone...and the stone fell into his pocket. He saw his greatest desire and achieved it. Why couldn't that happen again? Why couldn't the mirror give him whatever he wanted?
It makes perfect sense, if he ignores everything he knows about the mirror, everything the Headmaster told him about the protections for the stone - and why shouldn't he ignore what Dumbledore told him? The Headmaster lied to him about lesser things.
So he enters the room, sheds his invisibility cloak, blocks from his mind everything his friends would say, everything Dumbledore already told him, because Dumbledore is dead and his friends far away from this room, from the mirror, and they'd never understand.
He pulls the sheet off the mirror and it falls limply to the floor, sending a pale fog of dust into the air. Harry sneezes, and a part of him thinks it's completely ridiculous that here and now, before he commits a suicide of the mind, something so common can break the stifling silence.
Harry steps in front of the mirror, wondering what he'll see, though most of him doesn't care. He just wants to lose himself for a while, for forever, until all the bad memories and all the pain are nothing more than faded memories.
He sees himself first, smiling, his eyes bright and happy. Hermione and Ron stand on either side of him, his faithful bookends. Hermione's hair is as bushy as always, her eyes as warm and welcoming as hot cocoa on a winter's eve, her smile soft in the way it gets when she's holding hands with Ron. Ron's eyes are all crinkled up as he grins, and he has an arm slung around Harry's shoulders, murmuring something that makes Harry's reflection chuckle and Hermione laugh. Ginny and Neville and Luna are behind them, all of them chattering away happily.
Harry isn't sure how long he's been watching - long enough that, all unknowing, he's slid to the ground, sitting on the sheet with his knees drawn up to his chest, avidly drinking in the sight of himself and his friends - when the realization slowly creeps up on him that he's seen this before.
He's seen this very same image before, each and every day since the end of the war. Ron and Hermione flirting and jubilant, Neville quietly glowing with a newfound confidence, Luna swaying contentedly to music only she can hear, Ginny joking and bubbling over with joy.
The only thing keeping the dream from becoming real is him.
For the first time, looking in the mirror, he realizes that he's the one that needs to change, and that the rest of reality has already aligned itself to his wants and needs.
And he finally accepts that the mirror can only show the way. The rest is up to him - here and now.