On strength at the broken places

Read the whole passage and the comfort thins. Hemingway writes many, not all, and the sentence after it says the world kills those who will not break.

So this is not a promise that suffering strengthens. It is an observation that some people are stronger afterward, in the specific places that gave way, and that this is not distributed fairly. Taken honestly it asks less of you than the poster version: only that you notice where you did mend.