Your arm is the length it is and will naturally fall where it's going to fall, depending on the geometry of the bass in question. I normally anchor on whatever pickup is in the P position, but when I'm playing my Explorer, my hand naturally ends up on the bridge pickup due to the relative position of the bass when worn on a strap. I'm lazy and ignorant so I don't fight it, but if you want to because you feel you must have your hand on the "P" pickup, then you've got some training/practice ahead of you. There's no easy way to lose what it sounds like you perceive as a bad habit.
Presumably there is an actual, physical J pickup there, and not a phantom, imaginary one? If this is happening on a plain P bass, then the BC answer is obvious - get a PJ bass, problem solved