That definitely happens to me. A lot. It's annoying and frustrates me, and I don't know how to tell you to resolve the issue. My 'solution' is usually just refusing to let myself write the story until I've finished what I've already started, but that really doesn't work for everyone. I also do some (a lot ) of short fic to capture feelings/ideas so not everthing comes with a 5K word count or huge investment of time.