Everyone keeps telling the speaker "We understand," and the speaker has had enough. Most of the poem is a furious outburst: a string of questions asking whether anyone truly knows what it is like to cry at 3 AM, or to feel the hollow certainty of being alone forever. The specific, quiet details make the pain feel private and real, and the final question, shouted with a mix of punctuation, shows how close the speaker is to breaking.
The ending shifts the mood. The listeners can only manage "a small gulp," then look away and quietly convince themselves they understood. That image suggests they are protecting their own comfort rather than facing the speaker's pain. The mood is raw, lonely, and bitter, and the poem leaves us with the sense that the speaker's isolation has just been proved right: saying "I understand" is easy, but staying with someone's pain is harder.