Bhagavad Gita 2.14

Contacts with the world give cold and heat, pleasure and pain. They come and go; they are not lasting. Endure them, O Bharata.

मात्रास्पर्शास्तु कौन्तेय शीतोष्णसुखदुःखदाः ।
आगमापायिनोऽनित्यास्तांस्तितिक्षस्व भारत ॥

matra-sparsas tu kaunteya sitosna-sukha-duhkha-dah |
agamapayino 'nityas tams titiksasva bharata ||

Contacts with the world give cold and heat, pleasure and pain. They come and go; they are not lasting. Endure them, O Bharata.

Psychologists call this "distress tolerance". Titiksha means sitting calmly with a hard feeling, such as worry, anger or sadness, and letting it rise and fall without panic. However, endurance does not mean punishing yourself. Titiksha is about staying present while an emotional wave passes; it is not a demand to become emotionally numb. Furthermore, enduring pain does not mean you are forbidden from fixing the root of the problem (like reducing a crushing email load at work or leaving a bad environment). Finally, remember that enduring stress is physical work. Your body's stress-response system (the HPA axis) burns heavy fuel to get you through the day, meaning true endurance always requires deep, physical sleep to recover.