Too many to mention right now, but I'm reading Rilke's 'Letters to a Young Poet' right now (which is unexpectedly hilarious in places, as well as more expectedly being very good):

"To keep growing, silently and earnestly, through your while development; you couldn’t disturb it any more violently than by looking outside and waiting for outside answers to question that only your innermost feeling, in your quietest hour, can perhaps answer.”

"Perhaps many things inside you have been transformed; perhaps somewhere, someplace deep inside your being, you have undergone important changes while you were sad. The only sadnesses that are dangerous and unhealthy are the ones that we carry around in public in order to drown them out with the noise; like diseases that are treated superficially and foolishly, they just withdraw and after a short interval break out again all the more terribly; and gather inside us and are life, are life that is unlived, rejected, lost, life that we can die of.”