In the Museum of Ethnology

[Fearful Symmetry]

Thanks for posting this poem, I enjoyed reading it 

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In the Museum of Ethnology

A brush and ink drawing
is on display,
protected by glass
in the Chinese room.

I’m not sure that “in the Chinese room” is necessary. It might be worth cutting.
I keep wishing that there were fewer line breaks. Some would say that it’s a matter of taste, and they’d probably be right, but shorter line breaks (to me) give a poem an airiness that I didn’t feel here.

With its free-flowing mane
and feathery tail,
a horse in mid-gallop
is charging towards me,
glorious, wild,
straining to leap 
from the paper.

Knowing how much
you love horses and art,
I think to myself,
'I'll go to the gift shop,
that horse on a card
will look good on your desk,
I'll post it tomorrow,
might take just a week
to reach your address'.

Maybe another stylistic preference but I think italics could work better here instead of single quotation marks.

A split atom later
the space-time continuum trembles,
people and artefacts fade.

I like a “split atom later”, but I have a hard time taking the term “the space-time continuum” seriously where it’s so closely associated with science fiction movies. I think even just “space and time” would suffice.
“artifacts”?

I implode,
feel molecules rearrange,
hear the splitting of stone,
and a new vein of sorrow
cracks open 
in the granite rockface of grief.

What was I thinking?

For an almost moment forgetting
you no longer lived there.

Forgetting you no longer live.

With its free-flowing mane
and feathery tail,
a horse in mid-gallop
is charging towards me,
glorious, wild,
straining to leap 
from the paper.

I like the repetition in the last stanza, it nicely and effectively transforms from a poetic image of the drawing to an ominous image with emotional resonance. I think it might be more effective without repeating the last three lines, though, or possibly just leaving off the last line. At least for me, mentioning the paper again waters down the strength of the repeated image, which no longer feels like a literal description, but a figurative image targeted at the narrator.