One thing that would probably help this process is to clearly define the emotional arc/change you want your character to go through. Then you can orchestrate all the chapters and events that take place between the beginning and the end to make sure you never deviate too far from your intended direction. It also helps to have a character reference sheet to refer to.
Sometimes you don't have to plan out your characters' evolutions ahead of time, though it's usually helpful, but there should always be factors of who they are that remain consistent no matter how much they change otherwise, that allows them to remain the character you love.
These are the center points of your character, and you can change other things around them, but never the actual basis of their personality.
I also find that if you WANT to write longer stories, then developing backstory is super helpful. One, it helps you to have a stronger understanding of your characters so they can remain consistent, and also the more you know about their backstories, the more details you will find to expand upon, to keep the story from feeling like it's length is unnecessary.