Your shading of the bush ignores the volume of what your shading. It's all flat, and doesn't respond to things like leaves sticking out, or being behind things.
The one on the left was a done with a flat gradient, while the one on the right uses a round gradient. The one on the left looks more three dimensional because it uses the shape of the object to make the shapes of the light and shadow.
Maybe you can try ignoring the details of the object and just paint the light and the shadow in a general form until you get it looking right. Then you can zoom in and worry about the details after. That's what I did with my edit.
