I went through this with mine and you're right, it's very frustrating! Here are some tips that eventually worked for me:
* during diarrhea, stop any sweet feed it just irritates the tummy
* lots of roughage
* a 7 day course of psyllium (my guy had lots of sand in his gut)
* a probiotic / digestive enzyme supplement works but it may take several months
* keep the poo off of the skin - it can burn. Dawn dishwashing soap works great. Rinse well then put vaseline on as a moisture barrier to protect the skin. Don't forget to clean in the crack of a muscular butt, the underside of the tail and the back of the heels. I also shampoo the tail, rinse thoroughly and then braid it to help keep it cleaner longer.
* don't let chronic diarrhea go too long, even on a younger horse, electrolyte imbalances can affect the muscles (the heart is a muscle) -especially during warm temps when their body is under additional stress just to keep cool.
Good luck and keep us posted!