In the act of being creative we come up with all kinds of ways to give life to the ideas rattling around inside of us. Sometimes we have the time to fully develop and explore an idea when it springs out and wiggles around on the page in front of us. And sometimes we do not have the gift of time to develop these ideas.
I used to get frustrated by this, which would often lead to trying to finish something and rushing and ruining the piece I was just getting to know. Over time I have stumbled on this approach: stop when you're not sure of the next step. Put the art away or place it somewhere you can bump into it.
When an idea comes to you about what could be a great next step, even if it's just a whisper of an idea, it's time to dive back in.
Take a digital picture of the piece before you take the next step. This preserves the art you have created this far and makes it possible to come back and recreate the work if you need to. I find this process incredibly liberating. I don't worry about "ruining" something, or making a big "mistake". I simply take the next step in developing a work of art.