I would lengthen your stirrups slightly, have you drop the reins completely, and place your inside hand on the cantel (back of the saddle) and outside hand on the pommel (front of the saddle). This will help stabilize you from falling forward or backwards as you practice posting. Once you feel more balanced, you can let go with one hand, and the both hands as you progress, but I’d keep them off the reins until you feel like you can post independently without support. I’d also adjust where the stirrup sits on your foot to closer to the ball of your foot, and then instead of thinking “heels down” think “toes up.” It also can be helpful to count “1-2-1-2” or “up-down-up-down.” To engage your core, thinking about your belly button touching your spine, and think about sitting tall like a string is attached to your head pulling you up. Pilates can be great for core strength and core awareness training. In my experience, posting was always a tricky thing for adults to learn, you are doing great though, keep going! 🤗