An interesting thing often happens in painting, knowing when to stop! It happens at every level, beginner, intermediate or very advanced (usually you can tell the signs by this stage). I have been thinking about on the way home today and not for the first time.
I like to think of painting as a type of story with a beginning, middle and end. This simplifies and hopefully helps.
Knowing when to stop can be tricky when you are painting. Often I urge others to stop if there is a certain amount of 'fiddling' going on but this is sometimes necessary at the end of a painting in order to bring it all together.
Stopping too early is also not that beneficial either! Paintings can take a very long time and you may have barely started before you decide a variety of things and that it is not working!
Solution? Walk away, make a cuppa, turn it over and return a bit later (not too long though or you may never get back to it). Look at the painting with a fresh and not too critical eye. Appreciate what IS working, evaluate what isnt.
If it is not working as you would like, keep going just to see what happens, you may break through this barrier or you may find it wasnt to be, either way you will find out.