Speaking to someone they once loved, the speaker imagines a future where that person moves on. A new partner appears, maybe a better one, and the speaker becomes just "a memory of once that was good." The promises they made turn into things that belong to the past.
What stands out is the speaker's quiet loyalty. Even knowing they'll be forgotten, they say they will keep waiting, because this person was their "light" and a treasure kept hidden in the heart. The last stanza is the saddest. Years pass, the other person forgets, and the speaker is left with memories alone. Even their love is expected to fade, but only "with time," and it's described as a "yearning heart" until then.
The mood is gentle, resigned, and wistful. There's no anger here, just acceptance mixed with a stubborn hope that never quite lets go.