The speaker arrives among people who have fallen, whether in fortune, morals, or spirit, and admits an uncomfortable motive: "loving for advantage." Their love is not generous. It is a kind of using, and the poem lets that confession sit plainly in the first two lines.
The middle lines turn to a cook who "hunts / a constant heart." It's a strange, slightly unsettling image. A cook is someone who prepares meat, so the search for a faithful heart can sound like both longing and appetite. Love and hunger blur together.
The ending is the bleakest part. Lice infest the speaker's coat, a sign of dirtiness and discomfort, and then comes the line "me against me." The speaker is both the pest and the one being bitten, at war with themselves. The mood is stark and guilty, with a hard, self-lacerating honesty. Because the poem is so short, each image lands like a small sting, and nothing is softened or explained away.