One of the most powerful realizations in my life has been that you can not be afraid, or sad when you feel grateful. Compared to most people I know, I am generally calm and in control of my emotions and optimistic.
This has an effect on a lot of things, as I am extremely grateful for a lot of things. When something bad happens, I am often the first who looks for things to be grateful for which alters my emotions completely.
By being grateful for what you already have in your life: life itself, health, a house, family, friends, a partner, a pet, a job, financial income etc. at the first moment of the day, this influences the rest of the day and life in general. You make your choices based on your hopes instead of your fears.