I agree, watercolors can be very difficult! Especially because mistakes are so hard to fix. But it's like you said; practice makes perfect : )
The darkest colors of this picture is about 3 layers of paint, and I used very little water. You probably know this already, but the less water you use the stronger/darker the color will get. If you just make sure the paint is completely dry before you add another layer the colours should get pretty dark without much paint!