The heart is empty. It has gone downstairs to the street .... It walks sullenly by itself and will not give up its monologue.
It wants direction but reads no map. Run after it before you are sorry!
Imagine the heart not as something inside you but as a stubborn person who has simply left the building. That's the central trick here. The speaker says the heart is empty, then immediately shows it going downstairs and out into the street, where it walks "sullenly" by itself, sulking and refusing to quit its monologue, the private, endless talking-to-itself of someone deep in a mood.
The title, "Tangent," fits neatly: the heart has veered off from the speaker, touching them briefly and then heading away on its own line. It wants direction but "reads no map," so it wanders without purpose.
The last two lines change the tone. The speaker stops describing and starts urging: run after it, before you are sorry. Whoever is being addressed (possibly the speaker themselves) is told to act before the loss becomes permanent. The mood is gloomy and restless, but with a flicker of urgency, as though feeling is slipping away and still might be caught.