Mike Stone's three short lines describe a quiet kind of disappearing. The poem suggests that we don't vanish all at once. Instead, time changes us "subtly," so gently that we hardly notice it happening, until we have become our own memories. We stop being living people and turn into stories that others (or we ourselves) carry around.
Then comes the last line, which is the sharpest: "And then we are forgotten." It is flat and plain, with no decoration, and that bluntness is what stings. Even memory, the poem implies, isn't a safe place to end up, because memories fade too.
The speaker sounds calm, almost resigned, rather than angry or panicked. The mood is wistful and a little chilling, like watching a footprint on a beach slowly fill with water. The shortness of the poem adds to this: it ends quickly, just as the life it describes does, leaving the reader with a small, uneasy silence.