My teacher tells me to lean back slightly in my saddle

 DONT!!! grip with your knees! :P   presuming your walk and trot position is O.K, let your weight fall into the stirrups (dont push) heels down, toes in, sit ON your bottom shoulders back, look up and move with the horse - when I teach clients they expect this to happen first time, really it takes 4/5 lessons with and without stirrups for them to get the technique.  hope it helps x

grip with your knees and stay sat heavy in the saddle