FIDELITY AND PERSEVERENCE IN PRAYER
Mental prayer is basically no more than an exercise in loving God. But there is no true love without fidelity. How could we claim to love God if we failed to keep the appointments we make with him for mental prayer?
—Time for God, p.17

This is very important. When we start doing mental prayer we are not saints, and the more we do it the more we realize that fact. People who never come face to face with God in silence are never really conscious of their infidelities and faults, but when we pray, such things become much more obvious. That may give rise to a lot of suffering and the temptation to stop praying. We should not be discouraged at that stage, but should persevere, convinced that perseverance will obtain for us the grace of conversion.
— Time for God, p.35

Our sinfulness, however grave, should never be an excuse to abandon prayer, contrary to what we may imagine or the devil may suggest. Just the opposite: the more wretched we are, the more motivated we should be to do mental prayer. Who will heal us of our infidelities and sins if not our merciful Lord? Where will we find health for our souls except in humble, persevering prayer? “It is not those who are healthy who need a doctor, but those who are sick. I have not come to call the just, but sinners” (Mt 9:13). 
— Time for God, p.36

In an age as keen on freedom and authenticity as our own, an argument that comes up fairly often and may prevent people from being faithful to mental prayer goes like this: “Prayer is terrific, but I only pray when I feel an inner need. . . . To start praying when I don’t feel like it would be artificial, forced, even a sort of insincerity or hypocrisy. . . I pray when I feel a spontaneous desire for it. . . .”

The answer is that if we wait until we feel the spontaneous desire for prayer, we may end up waiting until the end of our days. That desire for prayer is very beautiful, and also unreliable. There is another motive for going to meet God in mental prayer that is equally meaningful and far deeper and more constant: he invites us to. The Gospel tells us to “pray always” (Lk 18:1). We should be guided by faith and not by our subjective mood.
— Time for God, p.31