Regardless of all the other helpful things mentioned, if you pull your horse’s mane, it could thin out. I had the same happen to my OTTB and I no longer pull his mane, but use a trimming comb. Consider this: https://www.smartpakequine.com/pt/solocomb-1626
I haven’t used this particular product, but I use something similar that does the same job without pulling. Best of luck and I hope this is the solution!!