Echoes of Wartime (гет)

I am a dead person pretending to be alive. - Astoria Greengrass
Notes:
alternative summary:
After the war, Astoria tries to cast a Patronus. Or, three times Astoria tries to cast a Patronus and one time she succeeds.
(See the end of the work for more notes.)
Work Text: A candle flickers and suddenly, the shadows are moving. My eyes follow them, the monstrous shapes growing larger, looming over me, the darkness sucking out all the light from the candle and the faint moonlight streaming through my window.
Sitting on the floor of my bedroom, I am paralysed with fear, hand wrapped tightly around my wand, knuckles white and straining. Heart beating fast, thumping loudly against my ribs, the thump, thump, thump growing more and more erratic, my breathing shallow, and I can’t get enough air. My mind sees only the black of the monsters, the crimson of blood, dripping, smearing, and suddenly, my hands are bloody and I scream.
My scream dies when all the shadows merge into one, wrapping around me, my whole body freezing, the window frosted over and my shallow breaths misting in front of my eyes. I can’t look away, I can’t close my eyes and I stare into the deep abyss of the monster, my mind as frozen as my body.
Dead, scarred, bloated bodies float in front of me, missing limbs, cauterized wounds and I smell the burning flesh. Bile and bottomless terror coil inside me as familiar images play themselves over and over again, like a broken record. As broken as my mind. Icy blue eyes, once full of life, stare at me, the dead, blank eyes of my best friend. Rona. They stare, piercing through my skin, they stare at my guilty soul. I don’t know what’s happening but Rona rises from the dead and points a finger at me, full of accusation. I can’t take it and I throw up, but nothing comes out, and I dry heave. My face is wet, full o