Holding on to an idea

Hi everyone, I'm new here and just wanting some insight into others' work.

Time and time again, I have the "ah-ha!" moment, where I think I've got it, a fantastic idea for a book. But always, it can be 20,000 words in or before my fingers even touch the keyboard, I lose faith in the idea and cast it aside. The doubt can be because it's a fantasy theme and I think people will deem it ridiculous, or because my plot twists don't tie up convincingly, or because my characters develop issues and don't know where they're going.

It's so frustrating, I've wanted to be a writer all my life but apart from twice, I've never finished a full length piece of work.

I recently read Stephen King's On Writing and I loved it, he says he doesn't plan his books or write character references and this was music to me because I find that trying to plan it all out before I start just makes me completely fall out with the idea.

Does anyone else experience what I've detailed, or have you experienced it and managed to get past it?

Any advice much appreciated.

Sophie