Why is it I start writing something I will be quick, and relatively short and all of a sudden I'm 20 pages in with no sight in end? I know the end and the start but the middle just keeps writing!!
One way to solve it is to plan in more detail. Don't think of just the start and the end but how the characters get from the beginning to the end. The more you plan ahead the less likely you are to stray from the plan.
The eventual story might not be shorter but it should read better when finished.
However I found a flaw with this when doing NaNoWriMo in 2003. I had failed in 2002 so I thought better planning would get the 50,000 words written in time. I had an overall theme that had a beginning and an end with several linked episodes in between.
Once NaNoWriMo started I began with the introductory story. But I had planned a possible 36 intermediate chapter/stories. After writing parts two and three it was obvious that 36 chapters would be far more than 50,000 words. I had to hack my plan drastically. Eventually I wrote 12 parts but I could have written 24 or more. Some of the 36 ideas were only variations on stories I had already written.
When I finished I thought about the original plan. I should have taken more time to prepare and should have included more detail for each part. Even so I could have written 100,000 or 200,000 words and still stayed within the framework.
But if just writing and see what comes out works for you? Why not?
I know your pain. Every story is a novel even if it was supposed to be a short. Write the long version then chop them up in some logical manner and make them all short stand alone stories.
As long as you enjoy the writing, who really cares about the word length, (unless set by a publisher), just write till the story is told...or is abandoned as stories are never truly finished. hehe.
Another way to solve it is not to worry about length as long as it meets Lit.'s minimum wordage requirement. You can always change the length in review--either up or down (mine always goes up in review), but even that should be based on content needs, not by worrying about the word count.