Matthew's poem describes depression as a kind of invisible agony. The speaker pictures someone in terrible pain who screams until their lungs burn, begging for help while people walk past without looking. The short, blunt lines and repeated words ("You scream," "You suffer," "alone") make the pain feel relentless, like a loop the speaker can't escape.
The turn comes near the end. The person finds they "were never making a sound." The scream they thought everyone ignored was never heard at all, because only their mouth and eyes were moving. That twist changes the meaning of the loneliness: the problem isn't only that others don't care, but that the pain stays trapped inside. The mood is bleak, isolated and desperate, and the final lines read like a flat, firm statement of what depression feels like to the speaker. The use of "you" pulls the reader into that silence too.