This poem is clearly about the loss of a loved one and the regret we feel or perhaps the hindsight we experience when looking back. We don't know who, the narrator has lost, but we could perhaps infer it is the narrator's mother.
The narrator goes back and forth in time, as s/he remembers the letter writer. We know that whomever the letter writer is she was ill, and yet her words pass along the joys of her everday life appreciation of the simple things (perhaps because she realizes how transient life truly is.) Her illness she makes light of, as there is little said of "cold and pain."
Surviving life after the death of the letter writer is clearly difficult for the narrator as s/he has chosen to come to the grave site to ponder a life that has seemed "hard and strange" during the "broken year", and yet the narrator indicates that s/he is perhaps ready to begin letting go of the past as s/he "tip[s] the envelope from which/drift scraps of borage, woodbine, and rue," over the grave of the one who had most probably clipped them and sent them in the letter as a loving reminder of home.