"Diet" reads like a snapshot of someone caught in a painful struggle with food and body image. The speaker covers the mirrors with black scarves because they feel stared down by their own reflection. The "monster" that leaves tears behind seems to be a harsh inner voice or self-image, and the speaker quietly cleans up after it, then slips out "into my own doubt."
The two "blanket" lines, comfort and suffer, suggest the speaker can't tell the difference between what soothes them and what hurts them. Then the questions pile up, and the body starts to feel fragile: dizziness, cold feet, the sense of bones cracking. The line "my weight is weighing me down" turns the speaker's worry about weight into something heavy and almost drowning.
The ending is the most striking part. The words "do / i / eat / today?" fall one by one down the page, as if even this small choice is slow and frightening. The mood is anxious, exhausted, and trapped, with no easy answer offered.