The speaker has lost track of who they are, and the poem shows this by listing everything they are no longer. Son, father, partner, employee: these are the roles that usually tell us where we fit in the world, and one by one the speaker crosses them off. Each line begins with the same words, "I'm nobody's," and the repetition makes the loss feel heavy and unavoidable, like a door closing again and again.
The feeling here is emptiness, mixed with confusion. The speaker doesn't say what happened, and the poem doesn't explain it, so we only know the result: a person who defined themselves through their relationships and work and now has nothing left to hold onto. The final line, "maybe I'm nobody," is quiet and unsure rather than angry. The mood is bleak and lonely, but also very honest, the kind of thought people often have in the middle of the night.