Someone is trying to keep another person going, one breath at a time. The speaker's instruction is almost embarrassingly simple: just breathe. At first it sounds calm and reassuring, even gently teasing ("See? That wasn't so hard"). The speaker promises to stay close and share the burden, so the person addressed doesn't have to face it alone.
But the tone slowly shifts. The pauses and ellipses multiply, and the reassurance gives way to the admission that things may be "hopeless." By the last stanza the calm is gone. The lines shrink, the punctuation stumbles, and "Please.. Breathe!" bursts out as a plea rather than advice. The speaker seems frightened, and the final "Please... breathe.." trails off with almost nothing left.
The mood is tender but tense, like sitting beside someone in a crisis. The poem doesn't say what has happened. Judging by the language, it could be panic, grief, or something more dangerous, and that openness lets readers hear it however they need to.