You need to accept the fact that watercolor has a mind of its own. It’s up to you to act at the right time to get the best result. Learning that timing takes a bit of trial and error but when you get it right you will know!

Understanding timing and using gravity to one’s advantage are the most underrated skills in watercolor.

As watercolorists, we are dealing with very delicate wet paper. Keeping the surface wet helps our paints to run naturally without disturbing the paper fiber. A major factor is avoiding unnecessary brushstrokes, this way you’ll get a pristine, clean color that floats on the top of the paper like a thin colored film. Light can then pass through, hit the paper, and bounce back. This is how you get the sense of illuminated color in watercolor.

But timing is everything. You have to know when to keep going or when to stop and let an area dry before making your next move. This time is often referred to as the "Golden Time" and it is the time between adding the paint to the freshly wetted paper and the time at which it begins to dry on the surface. If you look sideways at your paper if you can see the sheen it is okay to add more paint and different colours but when that sheen goes you MUST stop and wait for the painting to dry completely before continuing with the painting. 

With watercolor, color and value can be unpredictable, often appearing much darker than the dried result. When your paper is still wet, they will keep evolving and merging but as soon as the paper loses it sheen you will start to get hard edges. Sometimes you want hard edges and sometimes not, the main thing is that you become aware when they are likely to form on their own.