Reader Question: Letting Things Die

I am going to try this thing called Reader Questions for a while, since I have some interesting ones that come up from time to time.  Today's is about letting things die.  Prepare your cleavers!

How do you decide what stories or characters you are going to write about, and which ones you have to let die without being written?  I  have been working on a book for four years, and I cannot figure out how to make it work.  Should I just give up?

This is from Josh, whose last name I will withhold becaus