@juliepeelart if it is a full grown horse it looks maybe a little too small, but maybe it's a young horse. Where the tail meets the behind might be a little high. Think about maybe doing something with one of those front legs. 4 straight legs is less interesting.
If the girl and horse are having a bonding moment(?), then perhaps it should be facing her and she's putting her hand on his nose. Or maybe she's hugging it and we see her cheek pressed up against it's chest. I guess I'm saying make the relationship between the two more clear.
I love, love the color and overall atmosphere you have going on. That tree by the lake is absolutely gorgeous. Well done on the girl, as well. But I'm not sure what to make of the castle in the distance. Is that where she lives? It doesn't look friendly and I think it is hurting your composition by way of being so dark and stark looking especially compared that wonderful tree. Add to it the horse looking off to the viewers right and I'm kind of confused as to what's going on. Or maybe nothing is going on. I can't tell.
If the castle is benign I would knock it back by reducing detail and contrast and lighten those dark purples in the far left corner sky. The clouds in the middle left sky are pretty and interesting, however, and work pretty well with the tree. Maybe could use less contrast, but I don't know if you should mess with it and risk overworking that section. If it's an unfriendly place I would still knock back so it isn't commanding so much attention. I find my eye lingering on it and wondering what is going on back there.