Or would they go on aching stillThrough Centuries of Nerve—Enlightened to a larger Pain— (21-23)
When most people think of enlightenment, they think of the end of suffering. Not Dickinson. Here she uses an oxymoron (she loves those) to argue that some sad people may be enlightened by their grief; in this case, they wouldn't experience a Higher Truth, but a Higher… Pain. They'd be the winner of the Pain Olympics, they'd win the Guinness World Record for Duration of Pain, they'd… well, you get the idea.
Death—is but one—and comes but once— (27)
While in many of her other poems, death appears as the Be All and End All, in this poem, death is "but one" in a list of things that cause suffering. An important reason for death's being less important here is because it only occurs once. Other kinds of suffering—hunger, envy, depression—endure, they stay awhile. But death? Happens in an instant.