One of the most valuable lessons that I've learned from the writings of saints and holy people is to live in the present moment, rather than dwelling on the past or the future. I'm certainly still learning to put this into practice, but I find that I'm most peaceful when I focus on God's will for today, rather than worrying about yesterday or tomorrow. Reflecting on the past and planning for the future both have their benefits, but they can also be an obstacle to holiness: Dwelling on the past can stir up emotions like bitterness, shame, self-loathing, or an unhealthy longing for what used to be, while dwelling on the future can bring fear of uncertainty or an unhealthy longing for future. By focusing on the present, on the other hand, we are able to freely surrender ourselves to whatever experiences or responsibilities God has set before us right here and now. Through this surrender to God's will - which is the essence of life in the Kingdom of God - the present moment is able to touch eternity itself.

In this article, I'd simply like to share some quotes to highlight the value of living in the present moment, rather than ruminating on the past or future:

Jesus said, “No one who sets a hand to the plow and looks to what was left behind is fit for the kingdom of God.” (Luke 9:62).

[Jesus said,] “Do not worry about tomorrow; tomorrow will take care of itself. Sufficient for a day is its own evil.” (Matthew 6:34).

[W]e are not to borrow troubles from tomorrow, because that day too will have its cross. We are to leave the past to divine mercy and to trust the future, whatever its trials, to God’s loving providence. Each minute of life has its peculiar duty — regardless of the appearance that minute may take. The Now-moment is the moment of salvation. Each complaint against it is a defeat; each act of resignation to it is a victory. (Ven. Archbishop Fulton Sheen, From the Angel's Blackboard)

The secret of happiness is to live moment by moment and to thank God for all that He, in His goodness, sends to us day after day... As to the past, let us entrust it to God's mercy, the future to Divine Providence. Our task is to live holy the present moment. (St. Gianna Molla, from Saint Gianna Molla: Wife, Mother, Doctor)

If I did not simply suffer from one moment to another, I would find it impossible to be patient, but I look only at the present, forget the past and am careful never to anticipate the future. When we surrender to discouragement or despair, it is usually because we are thinking too much of the past or the future. (St. Thérèse of Lisieux, The Story of a Soul)

O My God, When I look into the future, I am frightened, But why plunge into the future? Only the present moment is precious to me, As the future may never enter my soul at all. It is no longer in my power to change, correct or add to the past; For neither sages nor prophets could do that. And so what the past has embraced I must entrust to God. O present moment, you belong to me, whole and entire. I desire to use you as best I can. And although I am weak and small, You grant me the grace of Your omnipotence. And so, trusting in Your mercy, I walk through life like a little child, offering You each day this heart Burning with love for Your greater Glory. (St. Faustina, Diary of Saint Maria Faustina Kowalska: Divine Mercy in My Soul)

“Because I love My Father, I do always the things that are pleasing to Him." Thus spoke our holy Master, and every soul who wants to live close to Him must also live this maxim. The divine good pleasure must be its food, its daily bread; it must let itself be immolated by all the Father’s wishes in the likeness of His adored Christ. Each incident, each event, each suffering, as well as each joy, is a sacrament which gives God to it; so it no longer makes a distinction between these things; it surmounts them, goes beyond them to rest in its Master, above all things. (St. Elizabeth of the Trinity, Complete Works of Elizabeth of the Trinity)

We have to learn to live in the Present Moment. We have to ask God: What are You calling me to do now, in this Present Moment? Not yesterday or tomorrow; but right now. God’s will is manifested to us in the duties and experiences of the Present Moment. We have only to accept them and try to be like Jesus in them. (Mother Angelica, Mother Angelica's Little Book of Life Lessons and Everyday Spirituality)

Every moment of life is new to you, and God gives you Actual Grave in that moment. It is different from Sanctifying Grace. If you are baptized and keeping the Commandments and loving your neighbor, then you are in a state of Sanctifying Grace. But God grants us the Actual Grace of this moment, not the grace of tonight or tomorrow, just the grace for this moment. So we mustn't project tomorrow into this moment, because God will not give us that grace now; He waits until it is needed... God does not give us the grace today to endure the pain of tomorrow. But if I am living in the imagined pain of tomorrow with the grace I have now, I will always feel at a loss. (Mother Angelica, Mother Angelica's Little Book of Life Lessons and Everyday Spirituality)

One of the essential conditions of interior freedom is the ability to live in the present moment. For one thing, it is only then that we can exercise freedom. We have no hold on the past—we can’t change the smallest bit of it. People sometimes try to relive past events considered failures (“I should have done this . . . I should have said that . . .”) but those imaginary scenarios are merely dreams: it is not possible to backtrack. The only free act we can make in regard to the past is to accept it just as it was and leave it trustingly in God’s hands. (Fr. Jacques Philippe, Interior Freedom)

We may spend our whole lives waiting to live. Thus we risk not fully accepting the reality of our present lives. Yet, what guarantee is there that we won’t be disappointed when the long-awaited time arrives? Meanwhile we don’t put our hearts sufficiently into today, and so miss graces we should be receiving. Let us live each moment to the full, not worrying about whether time is going quickly or slowly but welcoming everything given us moment by moment. (Fr. Jacques Philippe, Interior Freedom)