The Dichotomy of Control
A concept I discovered when reading about stoicism. Focus on the things that you can control and disregard what is outside of your control. Sounds simple and obvious, until you apply it to everyday life and realize that most things you worry about are not under your control - other people's actions, opinions, politics, most external circumstances, really. What you can control however is how you react to those circumstances - your thoughts, actions and words, for example.