Hmmm...I ALMOST qualify as a 'nurse with a horse'...I'm still just a nursing student, and don't currently own my own horse but I've been riding hunters and jumpers for 10 years and am just leasing at this point due to time and money constraints. It's good to see there are other 'horsey nurses' out there!
As to your problem, it's hard to offer advice without knowing why he reared ...whether he spooked, was upset or frustrated, or angry. I suggest consulting a trainer and his past owners, if possible. As for building your confidence, I would go back to the basics...lots of simple walk/trot exercises, taking it slow, allowing yourself to build confidence with the small things first, also allowing the horse to get to know you and build confidence with you as a rider. It might also help to take some lessons on an experienced, quiet school horse. This will allow you to work on your skills and confidence independently, without worrying about your horse's problems, then you can translate this confidence to riding your horse.
I've often suffered a loss of confidence when riding a new/different horse, especially a difficult or green horse, and I suddenly feel like I have no idea what I'm doing. When that happens, I make sure to ride a quieter, more broke horse at least once a week just to boost my confidence and allow me to relax and remember that I DO know what I'm doing
Good luck!! Feel free to PM or email me if you have any more questions, or just want to talk horses!!