It seems, at times, as if the painting has a mind of its own, and it wants to go somewhere other than where you think it should go.
I find it helps to proceed as if this were actually true. The painter as detective...
Be patient. It usually takes me 4 or 5 attempts before I can begin to reconcile what I envision and what is happening on the paper. Then it may take another 3 or 4 before I can distill it down to what the two have in common.
Simplifying the image is the most effective way I've found to remember what I wanted in the first place, and to discover what the image requires. After that is made clear, then you can embellish, tweak, distort, or otherwise personalize the painting until it tells you it's done.
Look for an image or a scene that resolves clearly into layers.
Identify the major shapes.
Make an over-simplified study.
Practice whatever looks tricky.