I’m sorry, but I don’t use any mantras, but, for example, when I meditate I try to notice when my mind starts to wander and gently bring attention back to my breath or gratitude ( if I do gratitude meditation). I do this technique in order to understand myself better, because after that you can understand what is on your mind or what is bothering you.
I like to write about how I’m feeling for the day. Things I’m grateful for, things that went well in my day, things that didn’t go well and then what I can do in the future if it happens again.
I try to imagine that I am grounded and frozen in this moment. I find that taking note of things around me distracts me more, so personally I like to take a breath to clear my head, and focus.
Gently refocus your breathing. Mantras are such a personal thing, I would highly suggest going to Pinterest or google and finding one that resonates with you.