Parasite.

You're not the monster in my closet
Nor the thing beneath my bed
No you're the monster in my dreams
That plays around inside my head.

You siphon out my energy
Yet you put a smile on my face
You drive me to the brink of crazy
Searching my mind for another taste.

You're a danger to my health
But I need you to survive
Without you in my head
I feel like I will surely die

You poison me more each day
With no mercy for my waning life
You'll consume me till I'm gone
Like a deadly parasite.