The idiomatic expression "to become tough" means to endure or tolerate a difficult situation, often implying that one has developed greater resilience and strength as a result. Full definition
Here's where the Switch becomes a tough sell. (tomsguide.com)
To be honest, preaching work life balance can become tough when kids get sick, workload piles up, and stubborn house chores don't go away. (splashofteal.com)
By the end of week 1, I felt very mentally alert and had enough energy to complete my normal tasks, although my runs were becoming tougher as the week progressed. (primalkind.com)