Focus on your seat, and don’t squeeze with your knees
Don’t grip with your knees. Pretend you are pushing your legs into the ground and relax your hips and legs and let them fall correctly and focus on sitting the rhythm
You could always rise to the trot with no stirrups it's harder but it's a work out. If your sitting try and move with the horse and use you core.
You can't just grab the horse with your legs... You have to contract your abdominal muscles and try to follow the same rythm as your horse while sitting trot. You should try with someone lunging you so you don't have to control your horse.