Personally, I also have trouble writing long stories. What I usually write, as my own creation, are short stories.

I've managed to write a couple of long stories before, but that was because I drew a plot diagram, and divided it into chapters. Then I wrote each chapter as if it were a short story, so when I got the whole story together, it came out as if it were a long story.

I got the idea to do this from a friend who would draw an idea map for his long stories and then start writing the parts he felt inspired by, when he had all the parts of the map he would stop writing. He would then go on to edit and assemble the pieces, as if it were a patchwork quilt. He ended up with a global review and edit, and that was how he managed to write long stories.

Personally, I didn't get very used to this way of working, but I tried it and I can say that it can work. But, I always recommend doing what works for us and what we feel comfortable with, as long as it is something we are doing for the pleasure of doing it and sharing it.