My filly had a very uncontrolable mane about the time she turned two. It wouldn't grow long and just seemed to stick out every direction. Also it was wiry. Since my old gelding, who also has the black mane and tail, had a thick stiff mane, I thought maybe it had something to do with the black hair. Wrong!
Try adding some ground flax or just a bit of corn oil to the feed (that is if the horse isn't too over weight) have to watch that figure! Also when it gets warm enough to clip and wash, under cut the mane if it's thick and make sure it's very clean then use a "leave in" conditioner. Don't be chintzy with it.