You are very brave, and I commend you for that.
You said that you are ok handling them on the ground? You could probably still consider getting a degree in communications and horse management and still go the therapeutic route. I don't know if you have ever volunteered at a place before, but instructors are never riding the horses, they are on the ground interacting with the students, only with the very occasional doubles riding, if there is extra support that the rider may need.
As for the fear, and I apologize for being blunt here, but I have seen a (rare handful) of amazing riders or trainers that had very bad head injuries from a bad fall that always had a fear of riding after. It is horrible to watch.
Having said that, I've also seen plenty of the same instances where the rider was able to come back into it over time. How much time? Nobody can really say or tell except yourself. Find yourself a bombproof horse to practice on and someone you really trust with your life that is horse savvy-whether that be your trainer or your best friend, just someone that is really good at making you feel great and confident, and have them be your guide.
Don't expect the world from yourself the first few (or many) rides either. Baby steps, take it five minutes at a time. End on a good note. Before you get off, make sure you have calmed yourself down and you feel safe and secure. If that means it takes you one lap around the arena of walking with someone by your horses' side and then ten minutes of just standing on the horse while practicing deep breathing to settle your mind, do it. You must start somewhere.
I apologize, I just realized how long this ended up being, but I am passionate about these sorts of things and you sound like you'd really like some support and help through this. Feel free to PM me if you ever feel like.
Good luck and vibes coming your way, you have a whole slew of people rooting for you!
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin