That's The Demon Poem by Dorsey Baker

Constantly!
Constantly!
Constantly!
you tell him
to go away
and leave you alone
but he just
keeps coming back
as you push your shopping cart
around in the grocery store
as you drive
down the busy street
as you feed
your baby its breakfast
as you talk
to your doctor
in the hospital room
as you stand
upon the shaky bridge
looking down
into the water
of the raging river below
as you sit in the darkness
all by yourself
seem like
you have no power over your thoughts
its like they have a
mind of their own!
that's the demon!
trying to prey upon
your weakness