What is a term for a motivational or inspirational phrase that one repeats to oneself? For example, in the movie Cars, Lightning McQueen psychs himself up by saying to himself: "I am speed...I eat losers for breakfast."
In general it is an affirmation, or a self-affirmation. This term would include other ways to affirm something other than repeatedly vocalising it, but that is a common form.
Colloquially, it might be called a mantra. In its original use, mantras would be objects of contemplation rather than affirmation, and repeated for a much longer time, but the colloquial use will immediately bring vocal repetition to mind.