The fulcrum of a poem is its turning point. It is the place in the poem where ideas that are in conflict with one another are resolved -- where one idea wins and the other loses. The fulcrum tells us what the author really means to say. In my opinion, the fulcrum of this piece is the last stanza. In that stanza, we find what the author really means by this poem.
Throughout most of this poem, Bishop is saying that loss is sad but that it is not really such a big deal. These are conflicting ideas -- does she think loss is sad or does she really mean it when she says that loss is not that much of a "disaster." We can't tell.
But in the last stanza, it becomes clear to me at least that she really sees loss (at least the loss of love) as a real disaster. In this stanza, she stops talking about things like houses and material things and starts talking about emotion. We see that the loss of her love has affected her very deeply and that it truly is (though she continues to deny it) a disaster.
So that's where I see the turning point. Up until the last stanza I can believe that she really means that loss is no big deal. After I finish the poem, though, I am sure that she means the opposite, at least when it comes to losing love.
In the eNotes link, please scroll down to a discussion of this particular poem.