Do not let the moment pass. To put it another way, watercolor painting is something like jazz music. You will jam and create music this moment and if the spirit hits you, you will never be able to replicate the same music a second time, similar maybe but not quite. As with jazz, when working with watercolor, you go with the flow. It could be tweaked yes, redirected yes, but the water and the pigment will not wait. Once the moment and the creative eye escape you, it escapes forever, or you anticipate another one because usually it does.
Let Loose. The best artworks ever created by man are those whose final results are not totally anticipated. Of course there is always the general idea. The final outcome is an idea too. But to be fixated on a final outcome is to create art that is too rigid and too frozen. The best poets and writers will tell you anytime that to write well is to start with an idea and then let the characters do the job and keep on writing what hits you at the moment. Use less of your brain and think with your guts. That way unexpected results happen. Beautiful results, because art is an emotional thing. Masterpieces were done that way. In watercolor it is the same thing if not more so.
But first you must work. It is the knowing and mastering of the rules first that will enable you to break the rules with appropriateness and masterful art. It is the constant practice. Masters of the art developed the passion not because the passion was always there. It is the constant desire to create better that immersed them in the discipline that made them good. When the time spent is sufficient, the eye and the hand coordinate more freely that judgments in proportions, colors, and styles are developed.
Watercolor painting like all mediums of art has its own challenges and difficulties. But with the mastery come the reward and the satisfaction that always happen sooner if not later. Watercolor a hard medium? Not actually just different.