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 in your control and there are things out of your control.  If you desire things out of your control and you don't get them, you will be unhappy.  If you avoid things that are out of your control and they happen to you, you will be unhappy.

Therefore, on the topic of happiness and contentedness, focus on those things which are in your control.  The Stoics teach that practicing virtue (exercising courage, temperance, justice and wisdom) is in your control and will bring you contentment.