I can't sit the trot what should I do?

The inability to sit the trot comfortably is the position challenge that frustrates more developing riders for longer than almost any other, and it is frustrating specifically because the sitting trot requires the simultaneous coordination of multiple physical qualities — a deep independent seat, a relaxed lower back, a following hip, an independent leg, and a soft upper body — none of which can be developed in isolation and all of which must be present together for sitting trot to feel anything other than a controlled bounce. The sitting trot is difficult for the specific biomechanical reason that the trot's two-beat diagonal footfall creates a vertical thrust upward into the rider's seat with every stride — a thrust that the rider must absorb through her own body rather than using the stirrups and the posting motion to manage it. The body that absorbs this thrust correctly does so through a relaxed following lower back and hip — the lumbar spine and the hip joints acting as shock absorbers that allow the pelvis to move with the horse's back rather than bracing against the thrust and bouncing away from it. The body that cannot absorb this thrust correctly bounces, and the bouncing of the rider's seat on the horse's back creates the secondary problem of tightening the horse's back in response, which makes the trot more difficult to sit, which makes the rider brace more — a self-reinforcing cycle. The longe line without stirrups is the most efficient tool available for developing the sitting trot seat. On the longe line without stirrups and without reins, the rider is freed from the responsibility of steering and the temptation to use the reins for balance, and can focus the entire available attention and physical effort on the single task of following the horse's movement through a relaxed hip and lower back. Even one or two longe line sessions per month produces sitting trot improvement that translates immediately into better sitting trot in regular riding. The specific physical correction that produces the most immediate improvement in sitting trot for most riders is consciously releasing the inner thigh grip. The inner thigh that is gripping the saddle — the most instinctive stabilization response when the horse's movement is challenging the rider's balance — creates a rotational force on the femur that tips the pelvis forward, tightens the lower back, and makes the hip joint rigid in exactly the position that prevents it from absorbing the trot's thrust. When the inner thigh is released — allowed to lie passively against the saddle rather than actively gripping it — the femur rotates to a more neutral position, the pelvis drops back and down into the saddle, and the hip joint opens sufficiently to begin following the horse's movement. Building the sitting trot progressively — beginning with very short intervals of sitting trot within longer periods of posting trot, increasing the sitting intervals only as each current duration becomes genuinely comfortable — develops the specific muscular endurance and motor coordination that sustained sitting trot requires without the fatigue and tension that attempting sustained sitting trot before the prerequisite development is present always produces. Start with five strides of sitting trot, return to posting, repeat. Build to ten strides, then twenty, then a full long side, then a full circle, over weeks and months of progressive work.