The speaker keeps returning to the same spot, standing before a mirror, and each return marks a later stage of life. At first he can barely see his face, which suggests a child who is small and still growing. Then comes a young person full of big goals, "naively full of optimism," as the poem puts it. Gradually the mood shifts. Dreams begin to slip, doubts creep in, and reality starts to settle on him. By the end he is looking back, wondering where the time went.
The repeated line "I stand before the mirror" works like a clock, ticking from one age to the next. What changes is the feeling that follows it. Early on there is excitement and hope, though also some dislike of his own reflection. Later there is disappointment and worry. The final stanza echoes the earlier "uncertain of what's to come" with "fearful," so the uncertainty has turned into fear. The overall mood is reflective and quietly sad, about how fast time passes.