"Your first three ideas are wrong."
This was a piece of advice that came from the production designer I worked with the most in my theatre days, which he got from one of college professors. While I don't think it's completely accurate, I do think it's coming from a fundamentally correct place. A lot of the time, the initial idea is flawed, and it takes some work and thought before you get to the thing that's actually going to work.
Now, he was talking about design work-- how you build something that will look and function the way you want it to on stage-- but the principle is the same as with writing.
Around that same time, we both worked with another playwright, and one of the comments made of that playwright was, "He has some really fantastic ideas. And some truly terrible ones. And absolutely no skill at differentiating them."
And that's the challenge in writing, isn't it? Looking at the ideas you have, and trying to crack which ones will work and pay off, and which ones are not worth developing. I think it's still something I'm working on, myself. Now, part of my process is a long germination period, where I go from a vague idea to building the roots of it, and then growing it out in outlines and finally writing the story. I know my output speed would seem to belie this idea, but you're seeing the end result of the process which started many years ago and is now bearing fruit. In many ways, the two trunked novels were a necessary part of the process of the planning and plotting of all the Maradaine novels. And my space opera project (that is currently shopping) went through so many changes that the only things surviving from the original concept are A. the name of the ship (and the ship focused on is completely different) and B. one character (who in original concept was a stand-out secondary character that evolved into the actual lead).
Now, I could have stuck to my guns and insisted that the original space-opera concept or the now-trunked novels were how I had to go forward... then I'd probably still be languishing as a writer.
That doesn't mean every idea is gold, or I've mastered figuring out which are or aren't worth my time. Just slow, steady improvement on that front. Always learning. Any writer who thinks there's nothing left to learn is just stagnating.