The speaker is dealing with despair by refusing to take it too seriously. In the opening stanza, despair is treated like "an old friend" and "an old mockery," something so familiar that it hardly deserves a second thought. It's just "another fact of life," so why bother?
The second stanza shows the strange gap at the heart of the poem: words of hopelessness are "screamed in pitchy voices / to upbeat rhythms." Pain is dressed up as cheerfulness, and the two clash on purpose. Then the mood turns shakier. Tears and laughter mix, and the speaker is "trying not to drown" while wobbling toward apathy.
The ending asks whether pain even counts as a burden if everyone just lives with it. The answer is a shrug: "Doesn't matter really." The mood is tired, darkly humorous, and a little defiant. The speaker doesn't find a cure, only a way to keep going: scream it out, then carry on.