Three thoughts:
1) Fill in the blanks:
Once upon a time there was ___. Every day, ___. One day ___. Because of that, ___. Because of that, ___. Until finally ___.
That's a complete story, of any word count.
2) What's stopping you--specifically, what is stopping you? I've ended stories for a number of reasons, but, for me, it generally boils down into 3 main reason: a) the story bores me b) the characters bore me c) I don't know where to go next.
When I know why I stopped writing something it helps me understand both what I can learn and change.
For instance, writing 10k words in a story where I like the plot and story, but I found the character uncompelling and hard to get into. My options then become interrogating the character to make them more interesting (exercise suggestion of asking them 20 questions), changing the character completely (exercise: To what? What would be more interesting in this story) or removing them (exercise: does the story work without them).
3) Do you architect or garden? (Pants or Plan, Memorialize or Outline, etc.) If you've been doing one, or mainly doing one and not the other, switch it up. Change HOW you write to change WHAT you write.
Last point I'll make is that no writing is ever really wasted. If something isn't working, leave it. At the very least, you learned something about writing and what you like/don't like. At best, you'll noddle on it, come back to it, tweak, adjust or take it in a different direction. Don't beat yourself up over