There's a certain Slant of light
The speaker appears to be deliberately ambiguous about this "internal difference." We're not meant to see any specifics because it's "internal," and therefore only really understood by the speaker. And yet even she struggles to define this difference and the "imperial affliction" that comes with that slant of light.
- Lines 7-8: "Meanings" are located in the "internal difference." We might scratch our heads over these lines, but the point seems to be that those internal differences indicate a difference in perception. And since meanings are often determined by perception, the two kind of go together.
- Lines 9-10: Meanwhile, "none can teach" that internal difference, which makes it the "seal Despair." So the understanding of this difference that's illuminated by that initial slant of light can only come from the person who possesses it.
- Lines 11-12: The "imperial affliction" comes from the "Air," which tells us it's everywhere, both inside and outside of the speaker. So that internal difference is also reflected in the outside world.