Thich Nhat Hanh has his own way of working with these phrases, which I wouldn't necessarily call mantras but maybe 'practice phrases'. He has a quite poetic approach in general, blending the repetition of phrases that express intent with mindfulness of the breath. But it isn't just mindfulness, it's a sort of invitation into a gentle, serene mindset.

I find this closer in function to the tradition of gathas rather than mantra practice. Gathas are verses that express the practice mindset in relation to countless specific situations in life. In some cases they were created as verse sections of the Nikayas, the canonical scriptures; in other cases they were written independently by elders or anyone with a poetic . In Theravada and Zen, and in some Mahayana sources, gatha verses offered that sort of blend of devotional feeling with practical application.

~

A gatha from a female elder in the Nikayas, on the occasion of falling down:

Wandering for alms —

weak, leaning on a staff, with trembling limbs —

I fell down right there on the ground.

Seeing the drawbacks of the body, my mind was then set free.

~

A modern gatha from Zen Master Robert Aitken Roshi", on the occasion of a summer night:

Hearing the crickets at night

I vow with all beings

to keep my practice as simple —

just over and over again.

~

You are right that Thich Nhat Hanh's style of practice is much better digested in the course of ongoing study in one of his communities, with the guidance of a teacher.

If you have the aspiration, it can be wonderful to practice with gathas or these kinds of practice phrases, but the concern is whether it becomes an intellectual process that takes you up into your head, or even a rote exercise that relies on the words rather than the deeper mind.

The meaning of the word mantra is "mind protection". In its basic form it is easy for almost anyone to learn and practice. You don't need much training, you only need to do it. (Though studying with a teacher is always of great benefit.) Performing mantra continuously protects the mind — protects you — not because of some magical power, but because it helps you to remain momentary: you don't mess with negative thoughts, you don't linger in negative mind states, you keep moving on; evil doesn't stick to you.