This story's got a lot going for it. The aphasia and the insomnia seem well done, and the effects on each of them – frustration, weariness, inability to move on – are convincing. But though Harry has given up, once Snape starts helping him he's positive about it, and works hard, whereas proud and isolated Snape does his level best not to be helped or to improve! The relationship that forms between them, with Snape the only one to treat Harry as a thinking, competent adult, and Harry grateful for care and teaching, and strongly wishing to help in turn, struggling with his communication limitations to do so, develops convincingly slowly, turning to affection, love and desire.
The ending's a bit rushed, but we've seen their problems being overcome, and seen both of them committing to being together (or looking like it, anyway), and valuing each other. We can be confident it's going to work out. (Though I'd have liked to see more of that!)
I liked it, and thought you depicted an original set of problems in a world so full of problems that no one has attention to spare for needs other than life-or-death, especially for people who can't – or won't – ask for specific help.
(But the appalling frequency of wrong word choice – not just typos – drove me crazy.)