Try alot of sitting trot without your stirrups. This helps alot! Without your stirrups will make you balance your self almost automatically becuase you will not have your stirrups to rely on. Sitting your trot along with that helps becuase you wont be over on your horses shoulders
Part of it is your mind set. Try thinking about sitting deep on your seat bones, having shoulders back, and connecting your spine with the horses. Your mind is probably thinking your straight, but in reality you can go back even further. If you can, have a mirror out while you ride or video yourself. Watching your own mistakes helps you understand quite a bit! Hope this helps!