Some of the principles in Stoicism which can help us develop mental strength.

  1. Accept what you can and can't control
  2. Don't let your emotions control you
  3. Live in the present moment
  4. Focus on things that are truly important
  5. Do your best and don't worry about the outcome
  6. Help others
  7. Practice minimalism
  8. Be grateful for what you have
  9. Be true to yourself
  10. Be courageous
  11. Be persistent
  12. Be optimistic but not naive
  13. Be resilient
  14. Be self-disciplined
  15. Be honest
  16. Be fair
  17. Be forgiving
  18. Be humble
  19. Be open-minded
  20. Be willing to learn
  21. Be patient
  22. Be kind to yourself
  23. Take care of your physical health
  24. Spend time in nature

P.S.: I don't remember where I noted down these habits/principles. If I find the source will surely update here.