Know What You Can Control and What You Can Not
You can achieve freedom and felicity once you realize this. The only things that you have direct control over are your thoughts, dreams, and opinion of events. Things that we cannot control are circumstances we are born into (such as Epictetus who was born into slavery), The anatomy you have, and your status among others. The common theme here is that you can control only what is internal, not external and controlled by the whims of others. Depression occurs when we obsess over externalities.