Death, and the source of self

Sometimes when I go to sleep, 
the thought of death comes to me. 
In a hundred years, we will be nothing 
but buried ashes or bones.

This thought feels disturbing and unpleasant, 
I ask myself what the course of that feeling: 
it is the fear no longer existing.

But what is the I that exists? 

If it is our body and physical senses, 
    I find it highly improbable 
    it will be other than gone. 
If it is our thoughts, memory and neurons 
  firing in our conscious mind, 
  it is likely like the body. 
If it is our unconscious being present in the now, 
  – I am not sure what that is. 
But if it is our spark of God, 
  our being one with the universe, 
  it is beyond our physical self, 
  and thus not dying. 

So the fear of death boils down to the question: 

  What is the I that I am?