Worrying about nailing the rising/posting trot are universal believe me, so relax, it takes a while to master.
First for me is taking the Mary Wanless, visualisations, and think first of tallness, a spring attached to the top of your head gently stretching you up, while weights on your heels draw your leg down.
Now while you are tall, you want to rise, but it is NOT an up and down motion, your head spring is not pulling you UP. Now you need a piece of elastic tied to your belt buckle, going away from you at a 45* angle, so you think of rising from there, practice this at a walk first, just getting the feeling of gentle movement. Then try at the trot, weight sinking in your heels, sitting tall and relaxed and rising from your belt buckle, all you need to do is clear the saddle, you don't have to get acres of fresh air between you.
You also can't possibly learn to rise in a good rhythm without your horse going forward in a good 1 2 1 2 trot, so a whip is very very handy, not abusive, it saves a lot of nagging and pushing on the horse.
This approach works for me, but there are others, best of luck