Meditations, Book 7, Section 31

Marcus Aurelius — The Emperor

Instead of following the example of worldly people — whose lives are nothing but constant change, whose minds shift as much as their circumstances — focus on this: how to make good use of whatever happens to you. Everything that happens can be useful. These events become raw material you can work with, if you care about doing things you can respect yourself for. Remember both goals as you face different situations. Look inside yourself. Inside is where all good things come from. It's like a spring that never runs dry, as long as you keep digging deeper.

Marcus Aurelius — The Emperor Original

Or rather leaving to men of the world (whose life both in regard of themselves, and them that they converse with, is nothing but mere mutability; or men of as fickle minds, as fickle bodies; ever changing and soon changed themselves) let it be thine only care and study, how to make a right use of all such accidents. For there is good use to be made of them, and they will prove fit matter for thee to work upon, if it shall be both thy care and thy desire, that whatsoever thou doest, thou thyself mayst like and approve thyself for it. And both these, see, that thou remember well, according as the diversity of the matter of the action that thou art about shall require. Look within; within is the fountain of all good. Such a fountain, where springing waters can never fail, so thou dig still deeper and deeper.