I think the individual pieces often work pretty well, but the relationship between them is off in a few places. Seems like the front shoulder and leg on the right side of the drawing are too broad and too high. Not sure if you were trying to change the horse's pose (e.g., make it look like the horse was stepping a little higher than it is in the reference). If so, it still looks too broad to me. Down the back, you have the rear end of the horse level with the head, when in the reference it's quite a bit lower. Draw a horizontal line through the eye in the reference and it would hit the horse's back haunch. In your drawing, it's higher.