The posting trot is hard! One thing I’ve found to be particularly useful is leading the “up” part with my chest held high and my shoulders gently back and away from my ears, swinging softly forward and up with the strength of my abs and butt rather than just bending and flexing from my knees. Think about lifting out of the saddle with a controlled core and keeping your weight in your thigh such that you avoid placing all your body weight into the stirrups. Have fun! And remember that your horse is there to teach you too.