That is a very nice piece you have made. I , too, like the mottled background. There's a lot of freedom and activity in the background, which almost draws my eye more to it than the horse. Also, if you were to take a black and white picture of this, the entire horse and the upper part of the background would be pretty close to the same level of dark gray. so, if you have the willingness to go back to it, I would suggest adding a few more lighter areas to the horse, maybe some more variation in light/color to give it more "shine" and to make it more interesting than the background. Maybe put more work into the area around the eye? And maybe make a few of the forelock hairs more distinct.
Small areas of increased focus and detail help the viewer "move" through the painting as they look at each area of increased interest and let the background support the foreground, not compete with it.
This is a bit heavy critisism, but I think you want to know what to do to improve, right? I would perhaps let this painting stay as it, and do another and as you go along you can compare the newer to the older and see your growth as an artist.