I have a confession.
I'm a perfectionist.
No, seriously, I'm that horrid type of perfectionist that if I can't get it RIGHT I won't do it at all! It's horrible.
Over the years, this has affected how I write. I now outline. Very detailed, scene by scene breakdowns of everything that should happen to make the story work. I also play with the characters in my head for a long while before starting that outline.
Then I start writing, often editing as I go. This always produces the same issue: I produce a damn fine scene. . . and then I burn out. It's hard to be 'done' all the time.
On my current WIP, I changed my pattern. I wrote down the cool ideas and made a scene, and I'll get back later to firm it up, add where it has holes, and will make it like the first scene (Oh, yeah, I still 'perfected' the first scene. Old habits and all that!) and present it to betas then.
So I'm here with a plea: Get it on paper. You have all the time in the world to fix what's already written, but ideas are often fleeting.