The only stories (including novels) that I have ever been able to finish are the ones where I already know the ending.
I don't have to know, chapter by chapter, what's going to happen, but I do need a sense of where I'm going. Science fiction author Joan Vinge once told me in an interview I did with her that the way she conceptulizes the process of novel writing is with the metaphor of a road trip. For her, novel writing is like knowing that you want to go to California with a bunch of friends. You don't know exactly where you're going (San Diego? Sacramento? Oakland?) or what's going to befall you on the trip (flat tire? in-car romances? too much drinking in Alberquerue that ends with a night in the poky?), but you've got your eyes set on California. Thus, there's plenty of wiggle room for that "magic" a lot of writers talk about where their characters do things the author isn't expecting them to, but you still have a goal, an end, in sight. This is exactly how I write.
I find, in fact, the more I've planned out my "trip," if you will, the tighter I write. I'm not a big fan of that bizarro advice often told to novice novel writers which involves colored index cards and a ridiculous amount of time spent organizing and brainstorming. But lately I've written to proposal, which is basically a synopsis of a book not written yet. A good synopsis is a sketch of the important emotional and action highlights, and knowing what those are going to be before I write saves me a lot of useless meandering down the backroads, if you know what I mean. I think of my synopsis as a kind of map to get me where I'm going faster and more efficiently.
Not everybody writes like me, though.
My bottom line feeling about outlines (and index cards for that matter) is that if you're having trouble writing, see if it helps. If you find the process a hinderance, stop. Anything that keeps you from writing is evil. This extends to index cards.