Stoicism

Never complain.

• If it's something you can control, change it.
• If it's something you can't control, don't worry about it.

Practice negative visualizations. Imagining the worst-case scenarios and other people in worse situations will make you appreciate what you have.

Do things as if you were doing them for the last time. It could very well be the last time you do them, so make it count. Do with intent.

Treasure every encounter, for it will never recur.

• a Zen Buddhist proverb

See difficulties as challenges, not obstacles. They are trials presented to you by fate, and you have to overcome them.