I felt a Funeral, in my Brain, (line 1)
What exactly does a funeral feel like? Is it painful? The person is already dead, so it can't be that painful, right? The first line of the poem raises a distinction between two types of feeling that will return later in the poem. These two types are sensation and numbness. Sensation is the sensitivity to feeling, and numbness is the opposite. The funeral suggests pain or trauma but also quiet and rest.
Kept treading–treading–till it seemed That Sense was breaking through– (lines 3-4)
The end of the first stanza highlights the speaker's sensitivity to feeling. Her "sense" breaks through some barrier. What is this barrier? It might be associated with the floor of the place where the funeral is being held. The continued walking of the mourners increases the speaker's tendency to sensation. She almost seems to be positioned below the floor.
Kept beating–beating–till I thought My Mind was going numb– (lines 7-8)
The repeated words in the poem like "beating" and "treading" reinforce a sense of unpleasantness. The sound has invaded her space, and she isn't happy about it. In the first stanza, the treading of the mourners causes her to feel more, but now the continued beating of the service makes her mind go numb or lose feeling. This idea is consistent with the principle that the mind can only take so much of anything, whether it be noise, pain, whatever. Does the numbness mean that she won't suffer anymore?