your leg is so far out in front of you that you must "lever" yourself up , which is both exhausting and requires you to have your upper body forward, which then means your butt has to be back, to keep the balance.
shorten your stirrup one notch and work on bringing your lower leg more under you, not so much in front of you. think of our heel kind of pointing toward the horse's back feet, and every time you rise in the trot, think of that heel to back foot connection, and your weight going down THROUGH your heel to the ground , right at the hrose's hind feet.
Practice pretend posting from the ground, standing , and think about rising with weight down through your heels, and your pelvis thrusting forward, while your upper body remains upright.
Also, like the others said, it just takes time. we can offer advice, and so can your instructor, in the moment, but it takes time on your part to build in the body memory and strength.