When I think of endings I always think of the last two lines of T.S. Eliot's famous poem THE HOLLOW MEN:
This is how the world ends
Not with a bang but with a whimper.
That's how I end up feeling when it comes to my stories - like a whimpering fool. My stories just limp their way towards an ending that always seems elusive and out of reach. In my effort to revitalize my muse I re-read what Orson Scott Card had to say about structuring stories in his two books CHARACTERS & VIEWPOINTS and HOW TO WRITE SCIENCE FICTION AND FANTASY.
To paraphrase, he recommends one first determine the type of story one is writing based on the MICE quotient (Milieu, Idea, Character, Event). Each type of story creates expectations in the reader of both a particular beginning and an ending. For instance, if one is writing a Character Story then it has begin at the point when the main character is unhappy with their current role in life and seeks out to change it. It's ending therefore has to be the point when that character does one of three things: finds a new role, willingly returns to the old one, or despairs.
That said, I believe my problem is not so much in figuring out an ending but in determining the type of story I am telling. I think most of my stories are a mix of Character and Event stories. I just need to determine which type dominates. Once I do, I think the ending will become clear in my mind.