It's not the cable, nor are you doing anything wrong.

The problem is that you live in New Hampshire. (or any hilly area where you shift often).

Cables fray and break at the lever because of metal fatigue. Shifting winds and unwinds the wire onto the lever drum. That constant back and forth flexing work hardens the steel, making it brittle and eventually the strands start to break. It's the same as when you bend a paper clip back and forth until it snaps, though slower (thankfully).

Since it's the bending that's the issue, how often you bend the wire by shifting determines it's life expectancy.

All cables that wind on pulleys or drums have a service life and in critical applications like elevators, they're replaced on a schedule to prevent failure.

Bike use is too variable to set a single standard for replacement, but you seem to have a sense of what to expect and might opt to waste a few bucks a year in early replacement to avoid the inconvenience of walking the hills.

OR, cut a 2" or so length from a direct pull spoke (no bend) and carry it with a hex key. If a cable breaks the spoke can be dropped through the barrel adjuster and secured to the inch bolt to hold the RD in the gear of your choice. With most RDs the barrel adjuster will allow manual shifting between a few gears, and the ft derailleur can cover the rest (sort of).