The speaker is remembering a cruel sentence someone once said to them in anger: "Go sell yourself to someone else." They insist they've forgiven it. They even understand why it was said, picturing the words coming out of the other person's mouth like vomit, followed at once by regret. But forgiveness doesn't make the words go away. They stay "branded" on the speaker's ribs, and in the speaker's mind they multiply into a jumbled pile of insults: needy, pathetic, impure, vain, weak, broke, conceited.
The mood is raw and quietly aching. The comparison to coughed-up blood suggests the words hurt the speaker as much as they hurt the one who said them. The ending turns the poem in an unexpected direction. The speaker asks how many people they might have haunted in the same way, and the broken, stranded "i" on its own line makes that question feel unsure and a little frightened.