Thankfully for Mr. Pratchett, you can't influence the genetic lottery or the luck fairy, so his is still valid advice. In fact, one could see "trust in yourself" et al. as invitations to "do or do not, there is no try", whereas "work hard, learn hard and don't be lazy" supports the virtue of scholarship as well as that of "know when to give up". Miss Tick is being eminently practical, and "do or do not", while also an important virtue, requires way more explanation before the student can understand it.