The poem is built on one repeated phrase, "One moment can," and each line uses it to show how much a single instant might hold. At first the lines feel almost like a general lesson about life. A moment can bring happiness or misery, build a future or wreck it, hand you a dream or leave you in a mess. The list is balanced between good and bad, which makes the speaker sound like someone thinking things over from a distance.
Then the last line changes everything. "One moment is how we fell apart" drops the "can" and the broad "us," and turns the idea into something that actually happened. The word "we" suggests a relationship, perhaps a friendship or a romance, that ended because of one instant. The poem's mood is reflective at first, then quietly sorrowful. The steady repetition feels like the speaker circling a memory, listing every possibility before admitting the painful one that came true.