The speaker is apologising to someone they hurt, and the poem keeps refusing to let them off the hook. It opens with a blunt, uncomfortable image: being found at a terrible time is like a dog-walker stumbling on a corpse. The first reassurance, "It wasn't your fault," is aimed at the other person, and it sets the tone of plain honesty.
The speaker admits they had prepared excuses full of "self-pitying bullshit," then drops them, saying the truth is less ornate. They recall the other person as vulnerable, intelligent and attentive, while they themselves were selfish. The detail of writing a poem on the person's thigh, a poem meant for someone else, shows how careless they were. The line about attention being a form of generosity lands hard: they were given it and gave nothing back.
The mood is regretful, raw and self-aware. The poem even calls itself a "failed attempt at elegance," and the repeated "that doesn't make it ok" shows the speaker knows an apology can't fix things.