I would start by checking your stirrups to make sure that they are even. Then while your cores engaged, keep yourself centered within the saddle and your shoulders square. Practice a lot of full seat flatwork and while you do so, keep your seat bones /shoulders even and don't forget to engage your core! It takes a lot of practice but eventually it becomes natural 🤗
Practice sitting straight in your saddle. Have some one watch you from behind and tell you when your off to one side. Also maybe check your stirrups are the same length just invade this is what is making you unbalanced. You can also do some no stirrup work to strengthen your leg to help you balance more!