I'd advise you to plan plan plan!
Obviously, when writing short stories, you are focused on creativity and spontaneity, which can be hard to plan for, but if you start with the essentials, you should be able to return to your original goals.
Most stories follow the same storyline: Exposition, Rising Action, Climax, Falling Action, Resolution.
Exposition can and should include much of what Janine said above about defining who your protagonist is, but you could also consider doing this for the antagonist. For example, if someone is facing inner-conflict, you might use a character's inner-thought or conversations with others to describe what that antagonist (which can be a problem, scenario, and/or person) means to the story. Does the main character find out that she has cancer? Does a wife find out her husband is cheating? Does someone get killed? Does someone have the urge to travel but not the means? Whatever the conflict is, you can spend time developing that before you get into the rising action.
Once you start with detailed exposition, you should plan alternate routes for your rising action, climax, and falling action. You may start one idea and decide to change it, which is all part of the writing process. Maybe the character who finds out that her husband is cheating could either stay with him and face those issues, leave him and start life on her own, keep it from their families, tell their families, ruin his life, confront the other person involved, pretend nothing happened, etc. How do these play out? What conversations happen? What is your character thinking? Where does she go? What does she do? How do her small moments in her day change, not just her big picture life? If you have multiple options, you give yourself more opportunities to help your writing grow and hopefully not run out of ideas. Again, you can always change it!
I hope this helps!
Alexis