One of the characteristics of watercolor as a medium is that it can be very unruly. This is code for: OMG. I can't make it do what I want it to do!!
You're not alone in that frustration. The trick is to work with the medium and to let it surprise you.
The knack of finding the perfect in the imperfect is a Japanese philosophy called wabi sabi. It's a good philosophy to have when working with watercolor.