I had this marked for Classic Horror, but it's not really a horror story.
Unless you count childhood neglect/abuse as horror.
It's an account of a, surprisingly stable after he told his story from childhood to enlistment, man who ends up in a mental illness facility because of a 2 line letter he wrote to his girl and punching his superior officer during a scuffle with the MPs.
It reminded me of reading When Rabbit Howls.
Or maybe closer to I Never Promised You Rose Garden.
Since it doesn't really fit any of my squares, I'm not going to slot this for now.