I wish my mother could ride, that would be an awesome way to bond with her.
I agree with draftrider, see if there is a rescue near you such as CANTER that needs people to come evaluate and exercise their horses. Or check into getting a job at a barn, often times they will be more than willing to work out a deal where you can get to ride.
I personally never had a problem with other people riding my horses, I found it educational to watch what my horse does while ridden from the ground. With my current lease I am a bit jealous that the owner's daughter is still riding but oh well, not my horse, I am just grateful to get to ride such a well trained horse.