I don't think the therapeutic riding volunteer is a good thing, due to the attention needs to be on the handicapped.
And most of this can be attributed I think to your horse handling skills, or lack of them in this case.
You need to be mentored by someone who can teach you how to handle horses. The more you are buffaloed? The more you will be.
Pay better attention too, to the horses ears, eyes, and body language, both that directed at you, and that directed at other horses.