The speaker looks back on a broken friendship and on the harmful "him" they turned to instead. The "him" is never named, so it could be a person, a habit, or something else that hurts them, and the poem leaves that open. The speaker says the friend never understood them, so they went to someone who "understood my moods" and forgave him again and again, even though he hurt them. They hid a surgery, therapy and a new disorder from the friend, not wanting to cause worry or receive pity, which they hate. That secrecy seems to have cost them the friendship.
The mood is raw, bitter and tired. The line "But you leaving killed me" shows that the friend's departure hurt more than the repeated hurt from "him." The thank-you near the end is sarcastic, and the short, flat lines build toward a hopeless conclusion: "The pain isn't worth it / Nothing is."