The curse in general, to be forgotten by everyone and unable to have friends, or hold onto any sort of life more than moment to moment and eventually to end up in the Unsung Realm.
The Unsung Realm in general a vast prison hell of entropic decay and cold filled with eternally imprisoned ponies wrapped in chains working vast machinery for an insane goddess.
Sing her song and become nothing. Sing her song and become nothing.
The fate of Penumbra after Comethoof was cursed, alone in a hostile quarter of the city in the middle of a war, no idea why, and pregnant with a child that is slowly killing her.
Too Bleak, Stopped Caring: Lyra's life is one endless, monotonously painful slog where every second that passes shovels another layer of angst onto the story. It takes a strong will to get through it. Even though the ending is best described as bittersweet, the story goes out of its way to emphasize how much pain it causes Lyra, making it one last punch in the stomach on your way out the door.