No dialogue/one or none actors.
He or she walks down the isle, looking at cereals/products. Some of the boxes read features of the character seen (ambition, creativity, etc.), and each ingredient are the events that lead up to these events. For example, creativity: nice art teacher. Anyway, the person begins to pile up the boxes in the cart. The cart begins to fill up too high. They begin to spill out, and the cart tips over, spilling the boxes all over the floor. The person stands there, looking at the empty cart and the characteristics that make up the identity of the character.
My second idea is the character walking through the isle picking up boxes of cereal. His/her mixture of addiction/loneliness/depression, as well as lack of consistency (basically, whatever you think of as the cause of the character's existential crisis), making the cart tip over and the character to look at what makes them up, and who they are.
How about cereal looking at all of the other boxes, nervously frightened by the lack of difference. Sounds of marching and chanting plague the room. The cereal looks amongst the copies of himself, placed like himself. The cereal falls on the ground, spilling out.
Good luck!