I rarely rely on any specific mantra; I think the key is less to rely on a catchphrase or a quote from someone else and instead really believe in who you are as an individual. Rarely are you going to find a “one size fits all” mantra that covers every tough situation.
(Inspirational quotes are great except people tend to spew these out as Instagram captions without really thinking about what they mean! I think that’s a little cheesy, and I sometimes cringe when quotes are taken out of context and become mere “quotes.”)
I think a better approach is just BELIEVE IN YOURSELF.
This is definitely much easier said than done. First, you have to actually set yourself up for success in such a way that you are able to believe in yourself. Second, and this is actually the more difficult part, you have to just do it, and actually believe you’re capable of whatever hard task lies ahead.
It’s far more important to rely on a deep-seated self-confidence and belief that you are, in fact, awesome, than to rely on a set of specific words. That said, having earned my Ph.D. at the University of Virginia is something of which I am incredibly proud, and that fact has (interestingly) helped me in more than one road race. I’m a full-time faculty member at UVa and often compete against others who run professionally for a living; I return home to post-race grading while they’re off to a massage… Here and there the thought that “I have my Ph.D. and I can do anything!” has served me quite well mid-race!
Find something about you that makes you feel proud and awesome, and use that to accomplish other goals.