Hi Julie,
I love the outfit and using natural light in general, but in your case you need better windows to do justice to your subject and yourself. There needs to be more light focused toward the head region that filters down, or at least that may seem more ideal. I have no budget for changing windows in my studio either, so I know my critique is not particularly helpful. Can you pose her again in a different space with better natural light; perhaps with taller windows?
I had the very same problem with my portrait of
Laura at Thirteen . The light was terrible in the client's home. In the same way, the window was not tall enough to get to Laura's head. I was a couple hundred milles from home, so I could not arrange for a different space to shoot, and had to work with what I had. I've never shared the reference photo before, but you will see what I mean. I had to paint her with some rearranged and adjusted light, out of my imagination. That is not always as easy as it may seem, to accomplish.
Perhaps Photoshop could help in your case a little, but I think it would be much better to shoot in a space with very tall windows. You have a good model, costume and concept. You just need the light to make this worthwhile.
Garth