I see a lot of motion in your reference picture that I don't see reflected in your rendition.
In your picture her left shoulder is down below her right, her feet are both firmly planted on the ground, and she's leaning slightly away from us.
In your reference her left shoulder is up, her left heel is raised ready to pin her around, and she's leaning towards us.
Here's an edit trying to capture that motion.
I used this action line as a guide for where to and what angle to place her limbs.
You've got some issues with her proportions. Her shoulders are too small, her butt's too big, and a few other little things. What I usually do to keep the proportions on target (or get them to the target) is look at how the different parts of the image line up with one another. For example, her left palm lines up perfectly with the middle of her butt, and the back of her hand would be touching her heel if they were at the same height. You can also compare different sizes this way; her left arm including her shoulder and hand is about as long, in the image, as her left leg including her foot all the way up to right below her but. Doing little checks like that while you're still in the beginning phases of the picture—and throughout the entire process—will save you a zillion bottles of aspirin.
I'm sensing a little bit of symbolism in certain parts of your picture too. Mainly her face and her spine. Try to draw the shadow that her spine creates instead of the spine itself. That's sometimes a very subtle difference, but it becomes incredibly valuable when drawing strange poses or lighting. The same goes or everything else you might be tempted to abstract into a symbol. I really think that's what separates an artist from a person with a pencil and paper.
That's about all I have. I hope something in here is useful to you!
