Is it a story? Or is it another good idea

How do we tell if we have a good story or a fantastic idea? Where do we draw the line about how to plot a new book or whether we should tread the waters of short fiction, where the odds of selling that grow smaller by the day?

The newest YA project I undertook seemed so under control. I had a great premise. It's supported by fact and my characters were so proud to join my stable. Then, right as the beginning was winding down, as I headed for the middle (the part of the story, I learned last spring while working a book fair at my local middle school, where the kids just wish we wouldn't write! They told me those parts were boring!), the story seemed to fold. A cure? I went back over the beginning, fleshed out the characters a bit more, and as it headed for the middle, added a bit more tension for the protagonist, something she (and I) never expected.

Will this go to the end? What will my girl do now? Is it possible to even construct a story where the character breaks the law but does it for what she thinks are good reasons, all the while struggling with serious inner conflict? Only time shall tell.