Commentary on Meditations: B6:41

If you set up as good or evil any of the things beyond your control, it necessarily follows that in the occurrence of that evil or the frustration of that good you blame the gods and hate the men who are the real or suspected causes of that occurrence or that frustration: and indeed we do much injustice through our concern for such things. But if we determine that only what lies in our own power is good or evil, there is no reason left us either to charge a god or to take a hostile stance to a man.

The beginning of the Enchiridion discusses exactly this point and is one of the most foundational concepts of Stoicism. There are things which are up to and and things which are not. If you desire things which are not up to you and you don't get them, you will be unhappy. If you seek to avoid things which are not up to you and they happen to you, you will be unhappy.

Therefore, on the topic of happiness and contentedness, focus on those things which are up to you. The Stoics teach that practicing virtue, such as exercising courage, temperance, justice and wisdom, is something which is up to you. And if you practice virtue, you will find contentment.