Here are a couple of quotes I've found thought-provoking, both from Alexandra Fuller's Leaving Before the Rains Come:
"'But we cannot live in the afternoon of life according to the program of life's morning, for what was great in the morning will be little at evening and what in the morning was true, at evening will have become a lie.'"
"'Although it's worth remembering it isn't supposed to be easy ... Easy is just another way of knowing you aren't doing much in the way of your life.'"
In the first case, Fuller's quoting Jung; in the second, she's quoting her father. For me, they are reminders that things change, and we change. We should keep questioning our settled notions, even our notions about ourselves and what we want and where we are going. And if things are tough, it doesn't necessarily mean we're doing it wrong.