No one can master the intellect unless he himself is mastered by the Spirit. For the intellect is uncontrollable, not because it is by nature ever-active, but because through our continual remissness it has been given over to distraction and has become used to that. When we violated the commandments of Him who in baptism regenerates us we separated ourselves from God and lost our conscious awareness of Him and our union with Him. Sundered from that union and estranged from God, the intellect is led captive everywhere; and it cannot regain its stability unless it submits to God and is stilled by Him, joyfully uniting with Him through unceasing and diligent prayer and through noetically confessing all our lapses to Him each day.
God immediately forgives everything to those who ask forgiveness in a spirit of humility and contrition and who ceaselessly invoke His holy name. As the Psalmist says, “Confess to the Lord and call upon His holy name” (cf. Psalm 105:1).
Holding the breath also helps to stabilize the intellect, but only temporarily, for after a little it lapses into distraction again. But when prayer is activated, then it really does keep the intellect in its presence, and it gladdens it and frees it from captivity. But it may sometimes happen that the intellect, rooted in the heart, is praying, yet the mind wanders and gives its attention to other things; for the mind is brought under control only in those who have been made perfect by the Holy Spirit and who have attained a state of total concentration upon Christ Jesus.
~St Gregory of Sinai (Philokalia, vol.4, pp.276-277)