You Can't Know What You Know Until You Know It
If I could share one important tip with writers, it would be to slow down. Take your time. Remember that this is not a race. And, this one is a biggie...you really and truly can't know what you know, until you know it.
I don't know why writers expect themselves to immediately know what they need to know in order to write their stories perfectly, when they haven't even given themselves the proper time to linger in the imaginal development phase. But, unfortunately, this does seem to be a chronic condition. Most seem to expect themselves to be able to skip straight to the polished final draft, where the story is already perfectly expressed, without all the floundering that goes on in the messy middle.