Not the end of the world
Definition 1 of 1
Pro Tip 1/3Stress Reduction
It's used to comfort and to suggest a problem is manageable, reducing stress over the issue. 
You got a low grade? It's not the end of the world; you'll improve next time.
Pro Tip 2/3Perspective
Helps to give perspective, signaling a setback is temporary and not catastrophic. 
So, it rained during your party - it's not the end of the world!
Pro Tip 3/3Emotional Tone
The phrase often carries a light-hearted or optimistic tone, even in disappointing situations. 
Missed the bus? It's not the end of the world; another will come.