Baby oil, coconut oil or apple cider vinegar
Use coconut oil! :) natural and works great!
You can try using a tail bag and maybe stop brushing it. Their are also some supplement so out their you can look into
I found that a change in my horse's feed made a huge difference in hair growth. Research and try an all-natural biotin-like supplement or a change in grain.
Rather than using strong store bought chemicals you can try making a DIY leave in conditioner for your horses tail that is all natural. I've made one with all different kinds of oils. Also be extra careful when brushing your horses tail!
Tail bag if that's possible, also just try bruthing it with a hard brush rather than a main and tail brush so that you're not pulling out the hairs and untangle knots with your fingers. Also if you're showing put in a false tail