The glass must stretch
Down his middle,
Or rather down the edge.
But he's in doubt
As to which side's in or out
Of the mirror.
There's little margin for error,
But there's no proof, either.
And if half his head's reflected,
Thought, he thinks, might be affected.
But he's resigned
To such economical design.
If the glass slips
He's in a fix –
Only one leg, etc. But
While it stays put
He can walk and run
Again, although a joke, this is a very serious predicament. It should be noted that Bishop’s father died when she was five, and her mother spent time in an insane asylum. Bishop was herself, trying to hold herself together through changing circumstances. 
 (Source:  Open Yale Courses.  )
The Gentleman of Shalott