My mom threatened me and told me she would make me ride with a board on my back if I didn't stop slouching 😂
It's going to sound weird but are your stirrups the right length, too long can make you fall forward & too short tip backwards?
I had that problem so i bought a thing that goes around your chest and shoulders and pulls them back if you try to pull them forward it hurts!
try riding with a crop behind your back. it's more of an exaggerated shoulders back then you need to have but it will help you get the feeling of where your shoulders and body should be