Wow I really dig the style dude  

I would have done an edit, no time enough.

In the ref, there are a few features you might try to copy.

Composition: the strongest item is the light diagonal from upper left to lower right. You can't just duplicate that from an almost square canvas to a 1x2, but you get the idea. Mostly, your bg upper right and lower left corners could hide in shadow. Also you don't need to show the whole tree, just a hint of a leftward curving trunk would be enough, and allow you to attach the swing ropes further to the right, and increase the rope angle (> more dynamism).

Light source in the ref is from upper left, as can be seen on the girl and statue at left. Your light is frontal, which gives you problems with shadows (and perspective as well because of the tree shadow). The ref light allows for better volume and contrast on the girl (main subject), better texture on trunk, leaves and grass (if needed). I also think you're taking shadow too litterally: you don't always have to show sharp cast shadows; and if you do, the usual angles of view would make them almost horizontal lines on the ground, 2-3 px wide. Shadows can independently have strong / low contrast (strong in the ref, low in your image) and strong / low sharpness (diffuse in the ref, sharp in your image).

The ropes use bright tones: I'd strongly recommend doing this! (you have them in the palette) You would then be free to use the darker tones in the bg.

i love this picture! i agree with r1k's comments, but overall really awesome! great job making the palette look natural

You're so right.  I'm so bad at perspective.  I'll see what I can do about a shadow.  Thanks.

looks pretty good but kind of wierd cause that swing looks like it would be about 5 feet off the ground when its not moving.  Arent swings ussualy lower to the ground so theyre easier to sit on?  Also you can easily see it as her sitting on the ground a little further back rather than on the swing if you just dont look at the ropes from the swing.  Putting a shadow on the ground from the swing might help that.