The first thing needed for understanding "Amnesia" is a thorough understanding of the literary allusion to the Orson Wells film Citizen Kane. A literary allusion is a reference to literature, mythology, film, legend, historical moments or periods, famous individuals, etc., as a means of developing a complex idea in one brief reference.
For instance, if I say to you that my friend is a real Adam Sandler, you immediately conjure up an understanding of what Sandler is like and apply that image to my friend. This gives you a deeper understanding of what my friend is like than if I merely said, "Oh, he's a funny guy, all right." Allusions develop a deeper understanding of a person, event, place, or situation more clearly and more quickly than an ordinary remark can do.
Citizen Kane is about a man whose life changed when sent to live with another family in his childhood. He left important parts of his early life behind. He ultimately became successful but psychologically tormented. The end of the movie reveals that his inner torment stemmed from having left his snow sled called Rosebud behind on that cold, snow-covered, snow-spilling winter day when he was separated from his life and family--and Rosebud.
Applying this summary of Citizen Kane to "Amnesia,"
I almost trust myself to know
when we're getting to that scene [in the movie]—
call it the snow-scene in Citizen Kane:
gives you an understanding of the premise of and the implied metaphor for the poem: there was a great moment of loss and separation in the life of the poetic speaker. [According to the norms of poetry, the poetic speaker may or may not be the poet Rich.]
The allusion is carried beyond this, though, and into the meaning of the title: "Amnesia." The allusion draws on a visual scene from the film and speaks of the curtain of snow that was falling over the boy who was being handed over to strangers while outdoors playing in the snowfall with Rosebud:
Becoming a man means leaving
someone, or something—
must the snow-scene blot itself out
the flakes come down so fast
so heavy, so unrevealing
over the something that gets left behind?
When the import of the allusion is applied to the poem, you can understand that, in the poetic speaker's memory, there is a falling shroud of veil covering the thing that was left behind. In other words, the Rosebud of the speaker's life is veiled over, is blotted out, by forgetfullness (amnesia) in the same way that the film scene is veiled over by falling snow,
where every flake of snow
is ... / its own burden, adding-
up, always adding-up to the
cold blur of the past
The interpretation of "Amnesia," then, has two central parts. The first is that growing up involves a "simple and pitiless ... / ... putting away of a childish thing"--a childish thing metaphorically represented by the sled called Rosebud of the film allusion.
The second part is that memory sends metaphorical snow-flakes falling over the memory of "someone, or something" left behind. The question is, "still, why / must the snow-scene blot" out "so unrevealingly" the memory of "the something that gets left behind?". The metaphors of Citizen Kane and Rosebud imply that the "someone, or something" left behind may be oneself or part of oneself because oneself is what one loves most.