It's falling forward from her head being down grazing. I would guess that it's getting thinner from you brushing it trying to get it to lay right. You can try braiding it in loose braids and see if that will get it to straighten back up. I'm going to try ragging my horses mane because the other horses out at my pasture have chewed a lot of her off, but have quit, and you can see a clear line of where it is quite thick at the top 5-6 inches, then thin the rest of the way down. I may just chop off the thin section, she grows mane quicker than any horse I've ever seen.