Prayer is both a gift of grace and a determined response on our part. It always presupposes effort. The great figures of prayer of the Old Covenant before Christ, as well as the Mother of God, the saints, and he himself, all teach us this: prayer is a battle. Against whom? Against ourselves and against the wiles of the tempter who does all he can to turn man away from prayer, away from union with God. We pray as we live, because we live as we pray. If we do not want to act habitually according to the Spirit of Christ, neither can we pray habitually in his name. The "spiritual battle" of the Christian's new life is inseparable from the battle of prayer.
Source: Catechism of the Catholic Church (Vatican.va)