Chapter 5 of the Manual of Epictetus

It’s not circumstances that annoy you, but your judgements about those circumstances. For example, death is not as scary as we think, because then Socrates would also have been afraid of death. No, it is your own opinion that death is scary that makes death frightening. Whenever we have setbacks, are stressed or grieve, we should not blame someone else, but we should review our own opinions.
Those who know nothing about philosophy blame others for their failures. Someone who has had a bit of training blames himself. And those who have completed their education blame neither others nor themselves.

Epictetus says here that it is not the events themselves that make us feel bad, but our judgments about those events. It may seem our emotions are inextricably linked to what happens to us, but according to Epictetus that is not the case. Unconsciously we stick an emotional judgment to everything that happens to us. We receive a compliment and immediately feel more comfortable. We are reproached and suddenly feel rotten. We cannot change anything about the event itself, we have no control over it, but we can certainly influence our opinion about that event. Epictetus tells his students time and again that they have to distinguish between what happens and what they believe. He emphasizes to them that following this principle helps to prevent them from being dragged along by unnecessarily disturbing emotions and thoughts.

Again, the students are expected to look closely at reality. Is that what happens to you really as terrible as you believe? How would someone else react to the same events? Is your judgment correct that this mouse is incredibly scary? Or should you adjust your opinion? Epictetus loves extreme examples and therefore does not mention a little mouse, but gives the example of death. The Stoic hero Socrates was sentenced to death by poison but was absolutely not afraid to die. He did not regard death as something bad. About his prosecutors he said: 'Anytus and Meletus can kill me, but they cannot hurt me'. If someone as wise as Socrates does not find death frightening, then we too, according to Epictetus, should not be afraid of death.

Finally, Epictetus points out that we also have a tendency to hold others responsible for our feelings. We like the persons who make compliments and dislike those who make reproaches. They make us feel good or just rotten. According to Epictetus, this really does not make sense. We are the ones who determine how we feel. It is our own choices, our own assessment of the event that determine how we feel. It is stupid and absolutely not stoic to blame someone else for your feelings. It does not stop there because in the end you are not really wise until you realize that neither the other nor you are responsible for what is happening in the world. You cannot change these events, but you do have complete control over what you want to think and therefore about your opinions. That means that you can not only think what you want, but that you can also feel what you want.