How would I describe the theme “hardships only make you stronger” in one or two words?

Solution 1:

OED:

Resilience:

5. The quality or fact of being able to recover quickly or easily from, or resist being affected by, a misfortune, shock, illness, etc.; robustness; adaptability.

1857 J. F. Smith & W. Howitt Cassell's Illustr. Hist. Eng. I. lx. 333/2 In their struggles with the ponderous power of England [the Scotch] discovered an invincible vigour, not only of resistance, but of resilience.

1977 K. M. E. Murray Caught in Web of Words xvi. 309 Although he still had surprising vigour of body and mind, he had lost something of his powers of resilience.

Solution 2:

"Builds character" is generally a phrase used when doing something hard that will make you a better person. Here's a WikiHow for 'building character.'

It's fallen out of use this decade, but it was used in Calvin and Hobbes by Calvin's father when Calvin protested chores.

There's also the slightly longer phrase on TVTropes, Misery Builds Character.

Solution 3:

I don't know of an exact match, but here are a few related words or phrases:

• "Trial by fire": doesn't imply that you are improved by the ordeal so much as that it proves your existing qualities.
• Similarly, a "crucible" is a common metaphor for an intense negative experience that would destroy lesser people, or destroy certain personality traits and leave only certain traits surviving.
• There are words and phrases that communicate patient endurance of hardship: long-suffering, stoic, and for that matter endurance.
• There are some that communicate an ability to survive hardship: resilient, indefatigable, indomitable (even, by metaphor, unsinkable, like Molly Brown)