My boyfriend suffered, still suffers with crippling anxiety and PTSD due to childhood abuse, much like you. Although he's never taken part in theraputic riding, he grew up with horses and has said many times that they are the only reason he is still here.
I take him riding with me sometimes when I'm at home on my bosses horses, and it's lovely to just see him light up. We went out for a gorgeous (but chilly) Boxing Day hack when he was at mine for Christmas, and although it was a sad day on the yard for all of us, having lost a horse that morning, it definitely took all our troubles away. Having spent the first 20 minutes of the ride crying uncontrollably, I felt so much better by the time we got home.
Boxing day with my three boys