Dang, I wrote a nice rambly post and I somehow didn't submit it.
TL;DR before I forget everything
Google 'how to write the middle of a story' and read the articles, there are heaps. A good starting point.
Finish bad stories too (finishing anything, even skinny storygames with meaningless choices, will help you improve).
Try planning more. Different people plan different amounts, so experiment!
In a similar vein, if stuck, go back and plan more.
Write some short stories. Writing the middle for these can still have you learn a lot.
Come up with choices by thinking of:
- What choice would cause this consistent setting (as Ninja mentioned) to change?
- What choice would impact character relations (insult a king vs praise a king, tell your friend you hate them vs care for them)?
- What themes is your story about? What choices would push the exploration of this theme in other directions?
- What would be cool to do in this situation?
Write! Force yourself to write even for 30 minutes during your free time (write or plan if you must). Revision can fix bad writing that might happen when you write and are not in the mood. Not writing = no middle, ever, so write!
Read and think about how things would be different if a character made a different choice. This way you can focus on just this without needing to do anything else. Reading in general is often said to help one write better too.
Conclusion
Maybe this shorter version will be more helpful...I think I missed some points, but there's heaps here anyway. Feel free to ask questions too, since I might've not explained some things properly.