Rather than not cantering, you probably need to put some ring time on that horse and work on balancing and body placement (of the horse and yourself) in a controlled environment. Especially out on the trail, you may find yourself in a position where you NEED to canter - or where your horse decides to canter - and knowledge is power.
And if your ribs are delicate, you might consider investing in a shock vest. The new ones are actually quite comfortable and not too hot and sweaty.
Are you riding English or Western?
You guys are making me so nostalgic for my hunter humper pony Chipper! He was a half Arabian pony that was the most fun a girl could wish for growing up. We were really successful on our local and state circuit. Every once in a while, I have a dream about him. My sister had her foot amputated a couple of years ago and has since gotten into dressage. She loves it and her Fresian (Waldo) and Thoroughbred (Willie). Her horses are the best therapy in the world. When you grow up with horses, I don't think it ever gets out of your blood.