I am so sorry for your loss. Nobody should have to witness loved ones in pain. You were brave and kind and showed your love by letting him go with the least amount of suffering possible.
It has been 5 years now since my heart horse left this earth and I still tear up regularly at the thought of never seeing her again. There are other horses but you just don't forget. It does however, get easier. Knowing you did right by your horse will make it easier as well. We should all be so loved as to have someone care about us like you obviously did for your dear horse. Hugs, and prayers to you during this most difficult and painful time.