A secret told
Exact repetition
The entire first stanza repeats verbatim. Dickinson rarely does this—the doubling enacts the secret's spread from one person to two.
Ceases to be
The passive construction removes agency—secrets don't get told, they just stop being secrets, as if it's inevitable.
appal but one
Old spelling of 'appall.' Only one person (you) suffers from knowing—until you tell, then it's two people suffering.
continual be afraid
Inverted syntax—normal order would be 'be continually afraid.' The awkwardness mirrors the discomfort of keeping secrets.