Someone is speaking about a love that has slipped away, and the opening lines make a strange claim: the speaker says they forgot they ever loved this person. They forgot the wanting, too, and every shared memory. But the poem quietly undercuts itself. By the second stanza the speaker admits to forgetting how to be honest, and "I still lie" suggests that the opening claims may be part of the lie. The line "I can never forget you, my vivid dream" gives the truth away.
The sadness comes from the final line. The speaker remembers everything vividly, while the other person has turned into "a fading memory." The love is one-sided, or at least no longer shared. The mood is quiet and wounded. There is no anger here, just a worn-out kind of honesty that slips out between the pretending. The short, plain lines make the speaker sound tired, as if they have said "I forgot" so many times they almost believed it.