One of the most helpful quotes for me has been one that I read early on:
"Motivation is an enemy disguised as a friend. Motivation gets you excited to the point you dedicate yourself to taking on a long-term activity, then it runs out on you.
Discipline is a friend disguised as an enemy. Discipline forces you to do things you don’t want to do, when you don’t want to do them, and it’s always there to nag you when you don’t do what you’re supposed to."
