Stress is the subject here, and the speaker's uneasy relationship with it. The opening image is a cart of broken boulders, heavy and awkward. Then comes a boast: "I bet I could carry more on my shoulders." It sounds like pride, or a refusal to admit the load is already too much.
The third and fourth lines suggest the stress is not all pointless. Among the worthless rocks are a few shiny pieces of gold, and the speaker seems to be guarding a secret. The poem doesn't say what it is, so we're left to guess that the burden is tied to something the speaker would rather keep hidden.
The ending turns sharp. "Relax, I am relaxed!" reads like someone insisting they're fine, which makes us believe it less. The last line, "Heading for another relapse," admits what the exclamation tries to cover. The mood is tense and quietly worrying: a person putting on a brave face while seeing the next collapse coming.