Fairytales are full of the contrasts between light and dark in the way they portray their characters. Fairytales are often grim and perhaps the antidote here has been what has become the "happy ever after" ending. Light and dark have to be in the right proportions. All darkness is just oppressive. All light would be blinding.
Look at the light and dark qualities of your characters. Are they in balance? What weakness in your character really lets them down? What virtue really makes them? How did they develop these things? Do they actively try to fight the weakness?
It isn't always appropriate for a story to end happily of course but the finish must be appropriate. I like to see stories end on a note of hope even if the finish is a sadder one.