Breastplate or nonslip pads
Two non slip pads. One under the saddle pad against the horse’s back and the other in between your regular saddle pad and the saddle
Breastplate, non slip pad, saddle fitter check
A breastplate could help, as well as a check by a good saddler. It could be that your saddle isn't fitting anymore because of muscle development or aging of the padding in the saddles cushions.
Simple breastplate. Or the straps on saddle pad.
Breastplate + non slip pad