My mantra is just rama rama rama. Meditation with a mantra helps a lot of you put in the work beforehand.
It sounds like you’re talking more about a motivational slogan, though. Mine is, “You don’t have to want it, you just have to do it.” It helps because it frees me from the tyranny of desire. I don’t need to figure out if I’m in the mood. I don’t need to trick myself into enjoying it. I can just do the thing and be done with it.
Relatedly, there’s a line from a favorite book, “Somebody has to and no one else will” with a similar vibe.