Ask HN: What do you do to get mentally stronger?

I have watched a fantastic documentary about Shaun Whites comeback [0], where an essential part of his daily routine targets mental strength. One exercise he does is bathing in freezing water for a couple of minutes.

I would like to know some other exercises, maybe you want to share yours :)

[0] https://www.youtube.com/watch?v=bFz52xVQIos