Bhagavad Gita , Chapter 2, Verse 14
It is one of the shlokas from Bhagavad Gita that tells you that nothing is permanent in this world.
Winters and summers are temporary in nature. Similarly, pain and pleasure are impermanent (anityāḥ). They will come (āgama) and go away (anityāḥ).
Tough times will come and go away. Learn to tolerate them without being affected by them. Nothing is permanent in this world.
The only thing that is constantly changing is change. So don’t worry!