Issues With Canter

I have been getting lessons for a few months now and I have always had issues with cantering, I just can never find the proper seat. I end up moving my legs and my feet constantly come out of the stirrup.

My trainer has always had me post trot with no stirrups in order to stregthen my groin muscles. However, I never use my groin muscles when I canter or any other gait (im only sore after no stirrup posting trots). She said that when she canters that she engages her groin muscles and the back of her legs to keep her seat when cantering, I do not do this. Should I lower my stirrups lower so I have to engage my groin mucles?

I have read about sliding back and forth in the saddle when cantering which I am starting to do. Am I using my groin muscles to aid in this movement, as in using them to forcefully push me back and forth in the saddle?

Thank you so much!

Sonds like the saddle doesn't fit you - legs are too far forward so perhaps stirrup bars are too far forward? Are you certain that the saddle isn't too far forward and sitting on the horses shoulder blade? That would cause the saddle to push the rider backwards in the saddle which pushes your legs forward.

Try attaching a picture of you cantering so we can see if you're in a chair seat - otherwise we can only guess as to what the issue may be.