Many writers seem to drift around in their writing, sometimes feeling it's like a warm bath that bouys them up and supports everything in their lives - and at other times that it's a cold maelstrom sucking them down to a miserable death. I tend to think that what water needs is a good pier to sit on, to extend this metaphor to absurd lengths.
If you don't have a frame or structure around your writing, you don't have any way to know how you're doing, when you've finished, or what you've achieved. Sometimes you may not even know what you're trying to do.
I encourage my students to write a sentence that states the intention of a story, novel, poem or flash and to keep it at the top of their work as a reminder of their intentions. If the piece starts to move away from those intentions it's time to step back and decide if this is actually a different piece of work, or whether you're going to rein it back into your original idea. Both are equally valid options, and both avoid the awful realisation that the story story that turned into a novella that became a play and then a novel is actually just a horrible mish-mash of unfinished thoughts and good ideas that went nowhere.