The speaker is tired of being told their suffering is "only in my head." The poem begins with that dismissive phrase in quotation marks, as if repeating what others have said, and then shows how mental illness can spill into the body. Anxiety becomes nausea, depression becomes fatigue, and anhedonia, the loss of the ability to feel pleasure, becomes weakness. The short, parallel lines work like a list of evidence, laid out for someone who refuses to look.
The mood shifts from tired frustration to something sharper. "Didn't think so" is flat and dry, and "But, go ahead. / Continue to doubt me" sounds almost like a dare. The ending is the most unsettling part. The speaker imagines a hospital visit as the moment when the doubt will finally be proven wrong, and "I'll be waiting" feels both cold and sad. It reads as a warning, but also as a quiet plea to be believed before things get that bad.