Why does the narrator confess in “The Tell-Tale Heart”?

In Edgar Allan Poe's short story "The Tell-Tale Heart," the narrator confesses to the murder because he is overwhelmed by a psychological breakdown. After killing and dismembering the old man, the narrator is visited by police officers who were alerted by a neighbor.

While conversing with the officers, the narrator becomes convinced that he can still hear the victim's heart beating beneath the floorboards. Driven to hysteria by this perceived sound—which reflects his own deteriorating mental state and psychopathology—he confesses to the crime to escape the unbearable auditory torment.1