I would look again at the photo (which I have no idea how to reverse digitally and am basically unable to do that mentally) and see where you need to increase the contrast a bit.
For example, the head needs to pop off the body a bit more. So , I might darken the area under the jaw, which recedes into space and look at the beautiful line of the neck muscles. This is an important element in the composition of this particular photo. It curves up and leads elegantly to the head, which then goes down the straight line of the nose. The ears have a "hard" prick to them, that shows the long away focus of the horse, a kind of "longing" for some far off place that is evocative, especially in the memorial of a now gone horse. Empahsise this with stronger line and darker color
I might have removed the halter, but since you have done it, and done it well, it tends to thave a bit too much importance. You've doen the eye and muzzle beautifully. Do no more to the back ground, it shoudl be LESS than the foreground so it doesn't compete.
Does any of this give you some ideas?