I cannot sit to canter!?

[OctobercasperGilbert<3]

I've been cantering for a long time now. I've cantered on various horses, mostly Moo, and I cannot sit to any of them! Once me and Moo had 2 beautiful canters in a lesson, he kept on the track and I kept a remotely still seat, but I don't know how I sat still and can't seem to do it anymore. I've been asking around the barn and all I get is 'relax your seat' which I try to do but I fail. Does anyone know any exercises which may help me sit still in canter? I try to move my hips with the horse and grip with my thighs but it never seems to help