1. Writing is never wasted. Sometimes I just write individual scenes involving characters or settings, without really trying to make them a story. Often, some of these scenes later tie themselves together in various stories, although they usually need to be revised to fit. I have a computer file called “Parts,” where I keep such unconnected scenes.
2. Related to the above, I started pretty early to keep a kind of "Encyclopedia" for each invented world I came up with. This would have brief descriptions of characters, races, plants, animals, cities, etc. It’s fun to do and also helps me hold the disparate threads of stories or settings together in my head where my unconscious can work on them. Some of these kinds of elements end up in my dreams because of that.
3. Remember that "you can write ugly" when you begin. The 'story' doesn't have to be anything publishable when it first comes out onto the page. Writing allows you time to fix all that stuff later. I find that the act of writing itself often generates a flow of creativity and things come out better than I would have thought when I was just 'thinking' of the story.
4. Related to #3, writing is really "rewriting." I've learned to enjoy it. I never have anything come out right when I first put it down, but I have confidence that I'll be able to fix it down the line.
5. A story idea belongs to you. Just because you’ve written it one way doesn’t mean you can’t rewrite it in another way. I’ve taken stories that I wrote early in my career and revised them based on experience, sometimes turning the core into a completely new story, and sometimes just an expansion of an original tale. Many writers do this. Poul Anderson and Louis L’Amour spring to mind. I have multiple versions of some stories, either with different endings, or just ones that were better developed as I grew in experience.
6. Reading a story is like flying over a landscape in a plane. Writing that story is trudging the ground, going up and down the hills, fighting through the underbrush, wading the streams. It's a lot more difficult but one experience can't replace the other. When I first started out, I sometimes took really strong scenes by other writers, such as Howard, or Bradbury, and typed them out for myself to get a feel for sentence length, paragraph length, etc.