The clinical literature describes ego death from outside. Subjects report the loss of the body's edge, the collapse of the line between observer and room, occasionally oceanic boundlessness. Then the session ends and the subject is driven home. Fiction has no such exit. It is obliged to stay in the room and keep writing.
The problem is grammatical before it is spiritual. A narrator who has lost the self must still conjugate. The prose must record a vanishing using the instrument that is vanishing. Three solutions exist. Refuse the first person entirely. Let the first person fail in view of the reader. Or set a clock beside the dissolving man and let the reader watch the hands instead of the face.
Tolstoy chose the clock. The Death of Ivan Ilyich (1886) takes five months to remove a magistrate from himself: first his office, then his household, then his opinions, then his name. In the final hours he goes looking for his fear of death and finds nothing where it was kept. "Where is it? What death?" There is no fear because there is no longer anyone to be afraid. From the bedside it is two hours of screaming. From inside it is light. Tolstoy declines to reconcile the two accounts.
In Fiction
Clarice Lispector, The Passion According to G.H. (1964), opens and closes on a dash. A sculptor enters a maid's room, crushes a cockroach in a wardrobe door, and spends the book being eaten by what she has seen. The dashes are structural: the sentence arrives from a place before the speaker and continues past her.
Samuel Beckett, The Unnamable (1953), keeps the pronoun as a formality. "I, say I. Unbelieving." A nameplate on an office nobody occupies. The voice does not stop, which is the horror and also the joke.
Identity fragmentation keeps its pieces. This keeps only the grammar. What continues, in each of these books, is the sentence — the way a struck room goes on sounding after the speaker has left it, and what the listener hears is the room.