Two short lines, and the whole poem hangs on a question. The speaker asks who they are to believe they are "only me." It sounds like doubt, but it's an unusual kind: the speaker isn't wondering whether they matter, they're wondering whether they are really as small and singular as they've assumed.
The word "only" does the heavy lifting. It suggests that "me" might not be a complete or final answer. Perhaps the speaker feels they contain more than one identity, or that they are tied to other people, to the world, to something larger. Or the line could be read more humbly, as the speaker questioning their own right to claim any fixed idea of who they are.
The all-capital letters give the lines a blunt, almost whispered urgency, like a thought that has just struck. The mood is curious and unsettled, but also quietly open. The poem doesn't answer itself, and that leaves the reader to sit with the question.