I think your problem is that you have way too many options to dip your brush in! Chose 2 blue, 1 red, 1 yellow, 2 earth tones. Reduce your palette to a minimum and your work and mixing ability will jump forward. You got a nice picture here, but way to many colours to get harmony. And the tree, should not be in the middle. For painting you could hold on when you begin the rule of thirds. Place the subject on one of the red dots