Writer's Fallout Shelter

Is it a problem when one intends to write a short story, but it becomes a novella or novel? I see people here who have difficulty writing novels, but my problem is the opposite. Once I start writing I don't know when to stop! I've had to use outlines for school projects, but the outline is always much more difficult than the actual writing. I'm spontaneous, especially when it comes to writing, and struggle with planning it all out the pieces. I have several stories on my hard drive that are long enough to be novels or novellas, but I don't have any in my portfolio right now and most aren't finished. Anyone have advice or suggestions?