Originally Posted by Trakhak

Agree that the bag looks to be the major factor in the premature failure of the cable.

Compare the last generation of STI drop bar levers with decent shift cable longevity, the ones where the cables come straight out the inner side of the levers and then bent very gradually down and back.

Many people, including me, choose to run the cable to the opposite side of the down tube, to increase the radius of the bend (and thus the lifespan of the cable) even further.

The shift other cable on the OP's bike might be ready to go, too.