I think of a mantra as a self-prayer. While originating from Hinduism and Buddhism as a phrase, sound or word to help one meditate and find focus, it’s not about religion. A mantra is about self-communication and healthy self-input. Think of all the negative things you think (and about yourself!) throughout a day, and maybe without even realizing it!
A mantra is a statement of hope, intention. A mantra is a conscious effort and reminder. It can be created to fit you, it can be changed, it can grow with you and be altered to keep up with your needs.
A mantra becomes a familiar statement that can be automatic and soothing, like prayer. Allowing a mantra to become second nature, like a saying or background reminder allows for easier access in your recall. With second nature recall, mantras are more easily able to be applied in times of stress or high emotions. It can be a simple familiar line of truth and intention to fall back on.
Sometimes a mantra helps in managing anxious, depressed or negative thoughts, and sometimes it is larger-scale. They can be as long or as short as you feel works for you. You can combine numerous aspects to incorporate a longer mantra, or keep things short and specific. Mantras can also incorporate more directive statements of intention, rather than self-regulation.