Most people expect a letter to bring something pleasant, but the speaker here sees it differently. Letters arrive "not like thanks but afflictions," as if each one were a small blow rather than a kindness. The speaker admits to fear, and then lists what the news might hold: a chance that has already been wasted, or someone's mind gone, perhaps a person slipping into dementia or confusion. These are losses that can't be undone.
The last image does the most work. Winter "bringing down timber in the forest" suggests bad news arriving the way a heavy storm topples trees: without warning, with great force, and out of anyone's control. The poem gives us no details about who is writing or why. That vagueness helps, because the dread feels general, something many people could recognize. The mood is anxious and bleak, quiet on the surface but heavy underneath, like the stillness of woods before something falls.