Much has been written about the Reiki Principles and it has been helpful to look at the original text from Usui Sensei (translated):
"JUST TODAY, don't get angry, don't worry, be grateful, work hard, be kind to others."
It should be noted that Usui Sensei was influenced by the Meiji Principles of the period which include many of the same ideals.
There are now many versions from various translations and paraphrasing. My favorite is from the Usui/Hayashi/Takeuchi/Takamori lineage:
Just for today, I will not anger. Instead, I will cultivate peace in my heart.
Just for today, I will not worry. Instead, I will develop trust.
Just for today, I will do my work honestly.
Just for today, I will not harm any living thing. Instead I will honor life.
Just for today, I will give thanks for my many blessings by practicing generosity.
Put into practice, these principles provide guidance and a place to start the day in a good place. To me the practice of living in the moment or being present is most difficult. It really helps to remember "just for today" and apply it to many life experiences.
If one reflects on these principles in times of stress - they can provide the perfect antidote to worry, frustration and fear.