Marcus Aurelius, before and after being made Roman emperor, made notes to himself on how to treat people, how to deal with stress, and why to stay honest.
He was called the last good emperor by Gibbon, and he seems to have followed 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 being free, it's very readable and pleasant.