how can i stop thinking about a very bad day..?

Answer 1

Write down your feelings! You would be surprised at how much better you will feel after you write down what happened and how it made you feel. I highly recommend keeping a diary/journal. I have for several years now 

Answer 2

That's why happy hour was invented 

Answer 3

Try a little music.  There is particular pice of music that is a sure-fire mood-raiser.

https://www.youtube.com/watch?v=49rS1zSmSYc

Answer 4

Exercise.  

Seriously,  go for a hike,  swim or run.