This from another web site, where they sell anode rods; the last paragraph may apply. I don't know how to change a softener to prevent "over softening" but I'm sure someone else here can offer some advice:
Recognize that the condition of your water is also a major factor and the quality of the water heater lining are two other factors in the life span of your water heater.
Anode rods that come installed with water heaters generally will be made of magnesium or aluminum that is wrapped around a steel core wire.
Water heater anode rods are generally screwed into the top of the tank.
Therefore, they are physically connected to the steel of the tank creating an electrochemical reaction, similar to that of a battery. The anode intentionally corrodes and the steel hopefully will remain intact. That is the theory of the anode rod.
When there's no sacrificial metal left on the anode, the tank can rust out.
Anode rods generally last about five years but it really depends mostly on the quality of your water and how much water travels through your water heater.
When salt is added to the water (such as when a water softener is used), anode rods can corrode more quickly. Water softeners can help reduce sediment, but anodes can corrode in as little as six months if the water is over-softened.