Don't worry yourself about bareback, I think just about everyone ends up with a slight chair seat bareback!
As for in a saddle, I would work a lot of 2 point. So start by using 2 point at walk, when you feel comfortable, start to trot, then canter. 2-point forces your lower leg to stablise or you will fall off! Once the lower leg is solid the rest of your body will follow.
When you can ride 2-point easily, instead of leaning forward, start trying to do it standing straight up. It's **** hard!!! I've been practicing for years and still struggle when on a different horse, but hell does it work at stabilising your lower leg, engaging your core and keeping you upright. Once you're really good at riding standing in your stirrups keeping totally upright, gently bring yourself back into the saddle. You'll find that you automatically stay more upright and rely less on leaning back to keep you in balance .