Even when you finish a story, you aren't finished. You have to go back and fix it. There's a quote (I think from Hemingway) that the first draft of everything is crap. Don't expect a final draft on your first run.

I usually plan something out and then just get it down on paper so I can have a first draft. I'll leave things that I know I'm not happy with because sometimes just forging ahead in the story can help you figure out what I needed to do earlier in the piece.

You might want to write your first draft of the current story you're working on and then put it away for a few weeks. Then go back and look at your story again with fresh eyes. (Maybe in between you can take a crack at a few of those unfinished stories).