Silent Shadow

I'm with you from the day your born
A shadow behind your heart 
As you grow, so do I 
Your soul, I am a part 
I'll nag at you, make you sick 
Though trust in me you will 
For this is why I do exist 
There's an emptiness I fill 
I decide on everything 
I'll make you think it twice 
I'm a helper with a need to give 
Your uncertainties advice 

What am I