My buddy Dave went through this exact thing with his pen clicking habit, and his trick was wearing a rubber band on his wrist and snapping it every time he caught himself reaching for the pen. Took him about 6 weeks before the urge faded, which is way faster than your 4 months with the stress ball. The key thing he told me was that the rubber band gave his hands something to do but didn't feel like a "reward" the way squeezing a ball did, so his brain stopped linking the habit to a good feeling. Maybe try swapping to something that's slightly annoying instead of soothing, since that seems to break the loop faster. Also he kept a tally on his phone every time he slipped, just to see the number drop week by week, and that alone kept him going.