My phone battery’s lifespan is getting shorter.
Your lifespan is your fate and will show you how long your lifespan is.
You did learn the reading of 寿, but the reading for 命 is an exception. When you think of a lifespan, think of how that affects everyone. Me, you... well, just those things. Combine them both into myou (みょう). That is who the lifespan affects. Myou!