It struck me — every Day —
The Lightning was as new
As if the Cloud that instant slit
And let the Fire through —
It burned Me — in the Night —
It Blistered to My Dream —
It sickened fresh upon my sight —
With every Morn that came —
I though that Storm — was brief —
The Maddest — quickest by —
But Nature lost the Date of This —
And left it in the Sky —
The metaphor in this is fantastic. The speaker suffers some great grief, and its pain is never far away. Let's start at the top.
The double meaning of "struck" in the first line is absolutely excellent. We say, of course, that ideas 'strike' us--a particularly vivid memory may strike us. So too for the speaker, she is struck "every Day" by her grief. But lightning also strikes; Dickinson takes this double meaning and runs with it. The pain of grief struck her like lightning, and moreover it strikes every day as fresh as "As if the Cloud that instant slit / And let the Fire through —". Beautiful imagery.
Notice the dash at the end of the fourth line. Dickinson often uses dashes when other punctuation might have been used. Here, the dash is not final as a period would be, rather I read it as an interruption. When the second stanza begins with "It burned Me", it is as though, for the reader, the very lightning that was just let through the cloud has burned her--that sentence had never yet completed when we begin the fifth line.
The second stanza emphasizes that the pain is with the speaker in the night, in her dreams, and in the morning--always with her, and always fresh.
In the final stanza, the metaphor of lightning for grief really does its best work. The speaker says that she believed that very intense storms, and by association very intense grief, soon pass, "But Nature lost the Date of This — / And left it in the Sky —". The grief is not only still with her, it is "in the Sky", looming over her, ever-present.
The pain of grief being represented by lightning in a storm makes it seem less a personal trouble and more a fact about the world. The speaker cannot simply give up the grief--cannot 'get over' the pain--because it is still there. What she lost is still lost. It is the world that is grievous. The storm, her grief, was not created by the speaker, and no more can she put it away that she can put the storm out of the sky.