What's the difference between the stories that you wrote with no intent on publishing them and the story that you wrote that was a part of the official series? Quality. The story that had no publishing intentions was leagues better.
Stop worrying about publishing. You didn't start writing to get published. You got into writing to escape. Your stories are not literature, and you know that your stories are not literature, so stop trying to make it into what it is not. Embrace the fact that your stories are not literature. Embrace what your stories truly are: words, sentences, paragraphs, and chapters of epic action packed masterpieces.
You still have your inner child, but you should not be suppressing your inner child when you write. When you write with the goal of getting published, this is seemingly what you do. When you don't, your inner child is free and you make gold. Forget getting published. Unleash your inner child. Write gold.
What you write is your world. Breathe it. Feel it. Live it. Experience it. Unleash it. Feel no pressure to enhance a world that you already created, that already exists, with your vastly improved writing skills. Make the same plot mistakes that you made when you started. Add the unnecessary adventure. Fight the unnecessary fight. Do what feels right. More importantly, do what feels write.