The turning around could be a training issue if she is used to riding one direction for an hour then turning round to go home. That's nappieness and can be trained out in time through things like not letting her rush home, taking her on circular routes and avoiding simply doing and about turn.
Or it could be physical, and she is getting sore after riding for a while. As she gets older she's not going to be able to go for as long, with things like arthritis starting to make an apperance. If your vet didn't pick up on soreness then get a second opinion, even if its from a non horse specialist vet. They could pick up on something the other vet didn't.