Originally Posted by Trakhak

Have to disagree. The shift cables in Shimano's early 9- and 10-speed STI shifters lasted far longer than the cables in the later versions with the cable housing routed under the handlebar tape. It's the abrupt, small-radius bend at the lever's exit point that's the culprit.

The OP might consider using a much smaller front bag or some other workaround. But installing high-quality, supple inner cables frequently is the simplest fix.