If you're writing a short story, it sounds like you've come up with the entire story in your head and are bored with it as you start writing so you shoot off to something tangentially related that you haven't already plotted out. If that's the case, I would recommend you try to write a story without outlining. You can use OSC's MICE quotient to give yourself a general overview of where the story will start/end and then just start writing into the great void and let your story go where it will.

If you're writing at novel length, I would guess that you haven't done enough world-building, character development, and general brainstorming about the story before you get started. Then when you get stuck and/or hit a slow patch, you're struggling to come up with something to write and so you get off-course. If that's the case, I'd recommend not writing a word until you've devised 1) all the rules of magic/technology for your story world 2) all your characters, including the villain, and their personalities/backstories 3) the main crisis of your story and a general idea of how you want to solve it, again without going into specifics. World-build until you're so excited about the story that you have to write it, and I suspect you'll have no problems getting past 10 pages.

Or you could just keep writing these little 10 page fragments until you've got a good dozen unrelated ones, pick up two or three at random and slam them together into one story to see what happens....

First, I need to know where the story is going. What is the point of my story? What am I trying to show?

Next, I need to get to know my character(s). How will he respond in different situations? My stuff is usually character based so I really need to understand the main character. I just wrote a story where the main character morphed after the opening. That threw the whole story off and I had to do a near complete rewrite. When I wrote the story with the character I originally intended it came out much better.

After that I usually just set the character down, point him towards the ending and see how he gets there. The ending is flexible to some degree depending on exactly what the character decides to do.

I don't know if this helps but I'd say to get your characters really set in your mind. If you really know them they won't drift, they'll just tell you to follow them.

This gets the story on paper without having to deal with telling the story properly.
You will know what story you actually want to tell. You can see where you need character and world development.
When you are ready to write, you have something usable as an outline to guide your story from beginning to end.

see if it works for you. I type 23 words a minute and I post story ideas based on this method and it takes me about a hour, maximum two, to write one out, just one quick sitting.

A boy with a haunted train is one idea. A girl who bought the train is another idea. The factory where the train was built is yet a third idea.

By themselves, they aren't enough to sustain a story, and that may be why things fade out after ten pages.

Try making two ideas work together (or against each other, if you prefer). Then start asking yourself questions about how the two (or more) ideas work.

For example, the girl bought the train for the boy and the boy discovered it was haunted. When the girl found out that she had bought him a haunted train, how did she react? Did she try to get the train away from him (out of jealousy? to protect him? what other reasons can you think of?) How did he react when he found out that a train the girl had bought for him was haunted? Did he blame her, or something else?

Then start asking what the characters want with regard to the train. And ask what they try to do. And then ask what can go wrong when they try to do things about what they want.

A larger than life character suffers insuperable struggles in dramatic contexts addressing a life-defining complication.

Note;

• A Character who becomes larger than life in the unfolding story due to the story's circumstances.
• Struggles as conflict, ie, life and death, acceptance or rejection, etc.; in other words Events.
• Contexts as settings, situations, and circumstances; in other words Milieu.
• Complication as the influx and efflux of antagonism forces that compel a character to act, to experience profound, high magnitude disequilibrium and seek to restore equilibrium, in other words a routine, Ideal, or value upset by the complication.

Incorporate a relevant theme based on the conflict for its meaningful and unifying power, evolve a moral and a message based on the theme and voilà,! a story presents or asks for more magnitude, depth, texture, emotion, or meaning development.