Try to post with one iron for each foot, each direction! Also, add a couple laps on your hardest side!! I hope this helps!!
Sit deep in your saddle with your back straight and do a sitting trot and walk then build up to a canter
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!
You should try stirrup-less and do lots of sitting trot. When you are in the canter, intentionally lean to the other side(the opposite side of where you lean) and maybe that would help :)