Maybe try simply always thinking it or writing 'shoulders back' on your arm. Also maybe having someone's else on the ground and saying shoulders back every time they come forward, eventually this will become a natural position for you. Also keep your eyes up, most of the time the rest of your body will follow - I find that helps with jumping
I ride with a whip against my back and threaded through my arms! Keeps arms and shoulders back, and then produced a habit of it 😌
I struggle with this too, Hannah where can you get one from? Also remember to look up, your head is heavier than you think and looking down will draw your shoulders and upper back forwards ☺️
Get a shoulders back bra! They work incredibly and you can wear me under a shoe coat for shows! :)
What I find that works for me is, sitting deep into the saddle and pressing my weight down into my heels. I always make sure I'm not balancing off the reigns and I have a good length of reign, not too long not too short.
Trying to keep the "shoulder, hip, heel" line may help you, imagine there is a metal rod keeping these three points in line. When I remember this, it always help me.
hope this helps you a little :)
You can try to put your crop behind your back by me it workt