Let me start with your first stanza:
My Name is David Simpson
I live at 1 3 4 4 Rosemary way
I live what you would call a normal life
But with every day that comes my sanity starts to astray

^I'm glad you introduced the speaker first, like that the reader knows who's story it is and it makes the whole thing more personal. I also liked how you'd make the speaker say that he thinks that others believe he's living a normal life, though what barely anyone knows is that he has a mental problem.

I walk around at night these thoughts in my head
I sit and watch people until they call to me
My hands start to shake as I get closer and closer
And what I do next is what my eyes of horror can only see

^It's good that you described the things that happen to the speaker, gives the reader an idea of what the speaker has to deal with. I liked how you didn't directly say what happens next, makes it seem mysterious and increases the interest.

I wake up the next day it feels like a bad dream
Theres so much anger, so much blood*
I cant get these thoughts to escape my venerable mind
The power, the control I wouldnt stop if I could*

^This stanza is filled with emotions. Listing things up in the verses marked with the * make them even better. I was able to feel how the speakers mind was filled with thoughts. Not knowing what to do or how to react after what happened.

The smell of the perfume the texture of her hair
I cant cope with the obsession thats controlling my brain
It all seemed so real one day it will be more then dreams
Is it an obsession that ive got or am I just insane

^This comes somewhat surprising you now talking of this woman, though you haven't mentioned her before. But i like how you question your current situation in the end. All in all i think it's a good piece.

You forgot an i in your title, it should be "obsession"
Not your best, but I liked the story behind it :)
check out your grammar a bit :P

It's terrifying to know that something is wrong in yourself, but not know exactly what it is.

A good write, the only critique I have is to work on the flow of your poem.

Other than that I encourage you to keep writing, and expressing yourself! You have all the potential.

Wonderful, 5/5!

p.s. Thanks for commenting, it is always much appreciated.