The shape of the mane and tail are really, really nice!
However, I would add some more highlights and lowlights. It looks very stringy so using different brush sizes and opacities can help stop that.
Another thing to do would be to try toning your horse, which brings out the muscle definitions and shine of the horse. I can see you've tried, but using black and a white to go over muscles and add details and setting the layers to soft light or overlay can really help bring the horses out!
Remember to add a shadow, and watch your lighting! The cutting also looks a little off on one of the hooves.