Four short lines carry a heavy feeling: the speaker describes being emotionally finished but unable to stop living. The heart, usually a symbol of love and feeling, has "decayed" long ago, which suggests a loss or hurt that happened in the past and slowly rotted away whatever was alive inside. Yet the "remains" won't vanish. Something lingers, like a memory or an ache that refuses to leave.
The last line sums up the contradiction: "I am dead yet existing." The speaker isn't literally dead. They seem to mean that they are going through the motions of life without really feeling it. The title, "Barely Breathing," fits this idea, since it points to a life reduced to the bare minimum.
The mood is numb, hollow and quietly bleak. There is no shouting or drama here. The plain, flat wording makes the exhaustion feel real, as if the speaker has gone past crying and now simply states how things are.