I tend to think about the "mantra" as another form of visualization or affirmation, but for the greeks.
Erase the false impressions from you mind, by constantly saying to yourself, I have it in my soul to keep out any evil, desire or any kind of disturbance - instead, seeing the true nature of things, I will give them only their due. Always remember this power that nature gave you
The Stoics believed that nature and logic held were the key to knowledge. And knowledge ultimately led to eudaimonia (happiness). And this - combined with a ethical life - was the key to virtue.
Every now and then, I need to remind myself that at this time, Greeks worshiped a pantheon of Gods. Zeus and Hera ruled over the other gods and man. The Stoics ignored the Gods when they could, because nature, logic and ethics held sway for them.
I think of the reality we face, and problems seem so much larger now. Then I remember that they actually had to defy the Gods to live this way.