I have trouble placing mortification in perspective. I have heard that St. Josemaría Escrivá advised, “Choose mortifications that don’t mortify others.” What does that mean?
Mortification could be described many ways, including “accepting reality as God sees it.” Mortification is a synonym for penance.
If, because of a bad temper, I decide to give up M&M’s as an act of mortification, that penance could reinforce my desire to be a less angry person (accepting reality as God sees it) or that penance could give me a false sense of security. How? I might think that having chosen the mortification, I am guaranteed to become less angry. Not so.
The quote you offered can illustrate this example. If I don’t really want to give up M&M’s but do so anyway, that decision may fuel my anger rather than decreasing it. Other people could suffer because I am not dealing effectively with my anger.
Mortification is about becoming more honest. It can reinforce a good decision but it cannot substitute for a good decision (for example, asking if my expectations of other people are reasonable).
When Jesus tells the story about the Pharisee and the tax collector in the Temple (Luke 18:9-14), Jesus does not criticize the Pharisee’s voluntary fasting and paying tithes beyond those required by the Law of Moses. Jesus, however, shows us that these penances did not lead this man to be more honest about God, himself and others.