Someone the speaker loves has gone quiet, and every hour of waiting to hear from them hurts. The speaker describes the heart as slowly grated and then turned into a soup that boils hard, with steam filling the chest until breathing becomes difficult. It is a homely, almost kitchen-like image, but it makes the ache feel physical and constant.
The mood is raw and a little desperate. Night seems to make things worse: "I crumble" is a short, blunt admission after the longer lines of suffering. The speaker also admits to a gap between how they look and how they feel, tough outside but "a marshmellow inside." They say they could cry but won't, preferring to endure the burning rather than lose control. That restraint makes the feeling seem even heavier. The final two lines drop the complaint and become a plea: if you can hear my heart beat, come back before I lose it. "It" could mean the heart, or the speaker's composure, and the poem leaves both possible.