Catechism of the catholic church ,paragraph 2014 says: "Spiritual progress tends toward ever more intimate union with Christ. This union is called "mystical" because it participates in the mystery of Christ through the sacraments - "the holy mysteries" - and, in him, in the mystery of the Holy Trinity. God calls us all to this intimate union with him, even if the special graces or extraordinary signs of this mystical life are granted only to some for the sake of manifesting the gratuitous gift given to all".

There are two kinds of grace 1) Sanctifying and  2) Actual

Sanctifying grace stays in the soul.

Catechism of the catholic church ,paragraph 2000 says: "Sanctifying grace is an habitual gift, a stable and supernatural disposition that perfects the soul itself to enable it to live with God, to act by his love. Habitual grace, the permanent disposition to live and act in keeping with God's call, is distinguished from actual graces which refer to God's interventions, whether at the beginning of conversion or in the course of the work of sanctification".

It’s what makes the soul holy; it gives the soul supernatural life. More properly, it is supernatural life.

Actual grace, by contrast, is a supernatural push or encouragement. It’s transient.

It doesn’t live in the soul, but acts on the soul from the outside, so to speak.  It gets the will and intellect moving so we can seek out and keep sanctifying grace

In its natural state, your soul isn’t fit for heaven. What you need to live there is supernatural life, not just natural life. That supernatural life is called sanctifying grace.

Second Vatican Council, Lumen Gentium 7: "In that Body the life of Christ is poured into the believers who, through the sacraments, are united in a hidden and real way to Christ who suffered and was glorified."

If sanctifying grace dwells in your soul when you die, then you can live in heaven (though you may need to be purified first in purgatory; cf. 1 Cor. 3:12–16). If it doesn’t dwell in your soul when you die—in other words, if your soul is spiritually dead by being in the state of mortal sin (Gal. 5:19-21)— you cannot live in heaven. You then have to face an eternity of spiritual death: the utter separation of your spirit from God (Eph. 2:1, 2:5, 4:18).

We attain this sanctifying grace by receiving sacraments. Our mortal sins leads to diminish or lose this Sanctifying grace.
Through the sacrament of penance (Confession), through your reconciliation to God, you receive sanctifying grace. But you can lose it again by sinning mortally (1 John 5:16–17).

Keep that word in mind: mortal. It means death. Mortal sins are deadly sins because they kill off this supernatural life, this sanctifying grace. Mortal sins can’t coexist with the supernatural life, because by their nature such sins are saying “No” to God, while sanctifying grace would be saying “Yes.”

Sanctifying grace implies a real transformation of the soul.
Once you have supernatural life, once sanctifying grace is in your soul, you can increase it by every supernaturally good action you do: receiving Communion, saying prayers, performing corporal works of mercy.

We must continually seek God’s grace, continually respond to the actual graces God is working within us, inclining us to turn to God and do good.

The peace of Jesus be with you !

Other references from "Catholic Answers".