Do HARD Things!
There is no replacement for doing hard things. Find them all around you.. challenge yourself… stairs instead of the elevator or escalator, cold shower instead of hot, no lifting gloves (for all that is Holy!), run instead of walk, Rx your training sessions (all of them, not just your cherry picked upper body bullshit), eat clean, fast for a few days… you get it! The changes these practices on repeat will make in every aspect of your life are irrefutable...
Period.
Put a fucking pin in it.