I don't have a mantra but, I do attempt to pay attention to my conversation between myself and the surroundings. How I act in it and how it acts upon me. I hope that by doing this, I can become more sensitive to the changes in both points and more quickly see when and how I should adjust and adapt.
Never had a mantra in the past either and always tried to be more reactionary then proactive. I see the mantra as a proactive reminder to stay focused on what matters. With working from home, Wife also working from home, and 2 kids there is always something that can distract me. So it can't hurt to take 5 seconds to remind myself to focus on my goals for the day.
It's good you have something that works for you!