Question

I just got my first draft on Emily wingspaninson's "There is a Certain Slant of Light" back from my English teacher. I had a hard time understanding the diction of the poem like "heft", “internal difference", "seal despair." I talked about how the slant of light signifies any natural phenomenon that make people depressed. My English teacher slashed the essay with big "No"s because she said the slant of light affected Nature. Can anybody please explain what the poem really meant and how I should approach the rewrite of the first draft? Ample appreciation in advance.