The following is taken from Divine Intimacy, by Fr. Gabriel of St. Mary Magdalen, O.C.D.
The spirit of mortification is really complete when, above all, we seek to mortify self-love in all its many manifestations. The Pharisee who fasted on the appointed days, but whose heart was so puffed up with pride that his prayer amounted to nothing more than praise of himself and scorn of his neighbor, did not have the spirit of mortification and hence was not justified before God. There is little value in imposing corporal mortifications on ourselves if we then refuse to yield our opinion in order to accommodate ourselves to others, if we cannot be reconciled with our enemies, or bear an injury and a cutting word with calmness, or hold back a sharp answer.
"Why," asks St. Teresa of Jesus, "do we shrink from interior mortification [of our ego, our will, and judgment] since this is the means by which every other kind of mortification may become much more meritorious and perfect, and may be practiced with greater tranquility and ease?" (Way of Perfection). As long as mortification does not strike at our pride, it remains at the halfway mark and never reaches its goal.
2. The true spirit of mortification embraces, in the first place, all the occasions for physical or moral suffering permitted by divine Providence. The sufferings attendant on illness or fatigue; the efforts required by the performance of our duties or by a life of intense labor; the privations imposed by the state of poverty - all are excellent physical penances. If we sincerely desire to be guided by divine Providence in everything, we will not try to avoid them, or even to lighten them, but will accept wholeheartedly whatever God offers us. It would be absurd to refuse a single one of these providential opportunities for suffering and to look for voluntary mortifications of our own choice. Likewise, it would be foolish for those in religious life to omit the least exercise imposed by the Rule in order to do a penance of their own choosing.
It is exactly the same in the moral order. Do we not sometimes try to avoid a person whom we do not life, but with whom the Lord has brought us into contact? Do we look for every means of avoiding a humiliation or an act of obedience which is painful to nature? If we do, we are running away from the best opportunities for sacrificing ourselves and for mortifying our self-love; even if we substitute other mortifications, they will not be as effective as those which God Himself has prepared for us. In the mortifications offered to us by divine Providence, there is nothing of our own will or liking; they strike us just where we need it most, and where, by voluntary mortification, we could never reach.
In order to arrive at sanctity, a certain specified amount of voluntary penance is not required of all; this varies according to the inspiration of the Holy Spirit, the advice of superiors, and each one's physical strength. All, however, must have that truly deep spirit of mortification which can embrace with generosity every opportunity for renunciation prepared or permitted by God.