The speaker has packed away pictures, notes, and a smashed bottle cap, things that clearly once mattered, and hidden them in a box under the bed so they never have to see them again. It hasn't worked. They can still "feel it through my mattress," as if the box were pressing up into their sleep and trying to seep back into their soul. The title seems to nod to the fairy tale of the princess and the pea, where a tiny object under a pile of mattresses keeps someone awake. Here, the pea is a whole painful past.
The second stanza shows the speaker's response: not facing the box, but piling on more. Old sheets, a rotting apple core, notebooks. The final line runs the words together, "moreandmoreandmore," so it feels frantic and out of control. The mood is anxious and darkly funny at once. The poem suggests that hiding something doesn't make it go away, and that avoiding it only makes the pile, and the discomfort, grow.