I have no technical advice at all. I can imagine your frustration with the blunt pastels and their limitations as to detail. This would frustrate me, too.
The ONLY thing I could say, is that when you wish the horse head image to kind of "dissolve", (in other words, it does't go all the way to the edge of the paper), it might look more natural if you had it dissolve by reducing the opacity of the color in a gradient, instead of coming to a rather abrupt edge. That kind of abrubt edge makes the head look a little like it's floating in the air.
The head itself is awesome!