Every moment... do what thou hast in hand with perfect and simple dignity, and feeling of affection, and freedom, and justice; and to give thyself relief from all other thoughts.
Marcus Aurelius, before and after being made Roman emperor, made notes to himself on how to treat people, how to deal with stress, why to stay honest. He was called the last good emperor by Gibbon, and he seemed to follow his own advice about avoiding luxury, as he spend most of his career camped near the border, supervising Rome's defences.
This was apparently never meant to be published, and it shows. It's pretty rough around the edges and occasionally repetitive, but I've also found it a constant source of motivation and stress-relief throughout my life. In a time where most people claiming to offer advice and motivation are blatant scam artists, this book is a treasure.
I would especially advise you to read the public domain George Long translation. Along with the translation being free, Long is one of the most readable Ancient Greek-to-English translators there has ever been.