What To Do When You Feel Like Quitting

I read a book 3 years ago that opined that the very things that make us human are limitations - pain, obstacles, age, weakness etc. These qualities are not found in the people tagged, 'Super Humans'. That's why you see the likes of Superman and Captain America doing impossible things, moving through barriers (and making it look easy). The thing is that it is not just uneasy, most of their stunts are impossible - I hate using the word Impossible but in this case it fits perfectly! The only reason why these Super Beings seem untouchable and all powerful - that is assuming they are real - is because they are not humans (at least not 100%).