When a world-class poet gets writer's block, what does he do? Well, he writes a poem about not being able to write a poem, which is exactly the kind of poem "The Circus Animals' Desertion" is. Tricky, eh? In this poem, Yeats thinks through the possibilities (and impossibilities) of Irish mythology, Modernism, and his own work, just a find a way to write a poem. And as it turns out, each one of them has all sorts of problems for our speaker.
Yeats is telling us that all writing comes from the heart, and anything that doesn't is not true writing.
In this poem, Yeats argues that writing about the heart makes for bad poetry. That's why he uses words like "foul" and "refuse."