Who am I?

I am your worst nightmare and I haunt your troubled sleep,
Virtual strangers we, but with a bond that runs real deep,
Although you do not know me,I've been there all your life,
And the pain that I inflict on you it cuts me like a knife,
And though I'm your worst enemy I'm still your loyalest friend, and to come and fuck up your whole day on me you can depend,
I'm the one you bare your soul to yet the one from whom you hide,
And the one who did destroy your esteem, confidence and pride,
I'm the one who takes all that you've got til you're stood there with f*ck all, but even so, I hope you know I'll catch you when you fall,
Do you know the one of whom I speak, if not please tell me why?
That you do not have the answer to.the question, 'Who am I?'
If you still don't know the answer, take a look and you will see,
That the one there in the mirror, looking back at you...it's me.