This poem is so powerful, to me. Dickinson seems to be speaking to both the timelessness and the dominance of pain. Pain is so overwhelming that, in the grip of pain, one cannot remember a time without pain... pain is all there is. And, the future seems to hold only continuing and relentless pain, where the sufferer's life and identity has been consumed by pain.
I think it's fitting that the poem personifies the pain; the person feeling the pain (i.e., the sufferer) is never referred to. I think that Dickinson deliberately wrote from the perspective of pain. Intense pain dominates the individual in such a way that it displaces everything else in his/her life.
In the grip of pain, everything else in your mind disappears... you are aware only of the pain. You are left completely in the moment, knowing nothing but pain (not when the pain began, nor if it will ever end). Pain can leave you with feelings of isolation, despair, and loneliness. When you've been in pain for so long, you oftentimes cannot remember a time when you weren't in pain ('an element of blank'), and you cannot see yourself ever feeling anything but pain.