Try using some extreme foreshortening photo references! Really exaggerating the poses and proportions can often teach us a lot about how to improve.
I hope this helps! Good luck :)
I really love your use of shade and light, this is a lovely piece. To improve hair, I would really focus on your use of light and trying to implement negative space. Something I like to do when drawing hair is to use an eraser to add shiny highlights.
I hope this is helpful! Good luck :)
