What truly, wholly belongs to us? It is our perception - our opinion - our attitude.
Indeed, we can prioritize everything in our life, but whatever we decide to focus on, we ought not to resent it. "Whenever you devote your attention to what is not your own, you lose what is truly your own" (v. 4, p. 53).
Epictetus also correctly points out that what we try to protect and cherish becomes a means for us to be enslaved. He says, "If I attache value to my poor body, I have given myselft up to slavery; if I attach value to my miserable possessions, I'm likewise a slave; for by doing so, I'm at once showing to my own detriment by what means I may be caught" (v. 24, p. 55). When we place importance on things that don't belong to us or are not in our control, we only torment ourselves. "But as a general rule keep this in mind, that it is we who cause aggravation to ourselves; that is to say, it is our own judgements that aggravate us and crowd us in this way" (v. 28, p. 55).
So what should we do? We should practice "maintaining always the same even temper," for this is what Socrates did.
This is why the discipline of assent is so important. We may experience a gut reaction to some event. We ought to pause; and in that pause, reflect on whether whatever it is that is trying to bother us. Is it in our control? Does it have sway or power over us? Most likely it does not. And instead, it is our perception that is holding us as slaves. Therefore, we ought to check our assumptions and change our opinion as needed. This is true freedom.