Why can the past not be forgotten?

Why does it creep in the night,
Searching and slithering like a shadow in the dark?

An endless cycle of memories I only long to forget.

An endless storm inside my mind,
Eroding away the barriers to my sanity.

"What will you have me do?"
"A bargain? A truce? What do you long for?"

Taunting and absurd, the voice echoes in my thoughts,
Like waves crashing upon the sandy shore.

"An end to the torment you lay upon me!"
"A torment lain upon yourself."

Slashing, booming, clawing, banging, hashing,
Destroying everything in the recesses of my mind.

Terror, horror, uncontrollable, and unrelenting,
A flood breaks through the gates at the sight of him.

"There is no bargain or truce to be made,
It will only end when you are nothing but a shell."

Despair, longing...guilt.

Why?