I find myself constantly dropping and picking up stories. I have the same thing as you when it comes to dropping a story. I usually don't know the end, I don't have a very strong plot, the story was going in circles, there wasn't a moral, and I get bored, frustrated, and depressed I couldn't produce anything. Writing shouldn't be so taxing on the nerves!
When I pick something up, it's usually a story I dropped a few years previous. I read through it and suddenly get inspired to twist and reform what I had written into something completely different. (I'm doing that to three books right now, I've dropped two again, both fantasy, and am working on a historical fiction presently.) My mom, who also enjoys writing when she has free time, told me that some of my stuff made better gold nuggets than stories. Not that it was bad, but she told me that my story could be a piece of a bigger story and that I didn't answer a lot of questions with just the nugget. Now I look at all my unfinished stories (which is pretty much all my writings) as possible pieces of stories that haven't even come to mind yet. ^^
Mom told me, "Maybe God hasn't given you all the pieces yet. You just have to wait for them. Sometimes He won't give you all the pieces of the puzzle right away. It may take time to discover them."

That made me feel a lot less guilty about my unfinished works.