There's a certain Slant of light
How we cite our quotes:
Heavenly Hurt, it gives us—
We can find no scar, (5-6)
The speaker's suffering is internal since she can "find no scar." And yet here it's described as a "Heavenly Hurt" that presumably comes from outside of the speaker. So we understand that her suffering is a kind of blend of both the outside and inside worlds.
None may teach it—Any—
'Tis the seal Despair— (9-10)
The speaker's suffering can't be explained or "taught" by anyone. It's rather something that can only be understood by the person who is suffering. So therefore it's a "seal Despair," since there aren't any quick fixes or easy explanations.
An imperial affliction
Sent us of the Air— (11-12)
And yet it appears as if the speaker's suffering is everywhere since it's an "imperial affliction" that's sent from "the Air." So here we see some more blending of the inside and outside worlds in which both appear to be the source of the speaker's suffering.