An Act of Perfect Contrition remits mortal sin on the spot, but requires a firm purpose of receiving sacramental confession as soon as possible and a promise to not receive the Eucharist until getting to the Sacrament. It should be noted that for an Act of Perfect Contrition to be efficacious it must be offered for ALL outstanding mortal sins at the time. One cannot offer contrition for one mortal sin while withholding it for another.
An Act of Imperfect Contrition will not remit mortal sin. If a person really cannot muster Perfect Contrition then the mortal sin will remain on his soul until he can get to confession. But, frankly, there should not be a problem mustering Perfect Contrition. If a person truly cannot offer a Perfect Contrition that would suggest a gross immaturity or mental problem, or other serious issue in which the person may qualify for diminished responsibility. Only God can know that. What we know is that theologically imperfect contrition leads one to sacramental confession, but it does not remit mortal sin.
Below I have copied quotes from the Catechism and an article by a bishop that explain all this:
Perfect contrition implies a fervent love of God. We are sorry for our sins because they offend God Who is so good. Mary Magdalen had perfect contrition. Her contrition was so perfect that she never sinned again. She followed Our Lord and was at the foot of the cross when He was crucified. Her perfect contrition and love were greatly rewarded, for He appeared to her on Easter morning. We should all try to imitate Mary Magdalene's contrition, arising from sorrow at offending God.
Perfect Contrition
Our contrition is perfect when we are sorry for our sins because sin offends God, Whom we love above all things for His own sake.
"Wherefore I say to thee, her sins, many as they are, shall be forgiven her, because she has loved much. But he to whom little is forgiven, loves little.' And he said to her, 'Thy sins are forgiven.' And they who were at table with hi began to say within themselves, 'Who is this man, who even forgives sins?" But he said to the woman, 'Thy faith has saved thee; go in peace' " (Luke 7: 47-50).
1. This contrition arises from a pure and perfect love of God. If we have a perfect love of God, our contrition for sins will be perfect. It ought not to be difficult for us to have a perfect love of God. We generally love our parents not for the food and clothes they give us, but for themselves, because we see their self-sacrifice, their unselfishness, and other good qualities.
Thus we shall be sorry, not only because we fear punishment or dread the loss of His gifts, but because we offend the good God, to Whom nothing is more evil than sin. If we can love our parents spontaneously, not for any reward we expect or punishment we wish to avoid, why can we not love God, Who is infinitely more lovable than our parents? If we love God spontaneously, because He is lovable in Himself, our love is perfect.
2. It is easy to make an act of perfect contrition if we sincerely love God. We can excite ourselves to it by thinking of the Passion, of how good God is, how many favors He has granted us, and how ungrateful we have been to Him in return for His goodness.
By thinking of God's gifts, we realize a little the goodness of God and His worthiness to be loved for His own sake. We then feel sorry for having offended our Benefactor by the sins we have committed.
3. If we happen to be assisting at a deathbed, and no priest is available, we should help the dying person make an act of perfect contrition.
The father of a family met with an accident and was at the point of death. The youngest child, who had recently made his first communion, saw that his father would die before the priest could arrive. He therefore took a crucifix from the wall, and holding it before his father's eyes repeated aloud an act of contrition. Tears filled the dying man's eyes. He died before the priest arrived, but his act of contrition washed his soul clean of sin.
4. We should form the habit of making an act of perfect contrition as often as possible.
It is only necessary to raise our hearts to God in pure love, and say some such words as: "O my God, I am sorry I ever offended Thee, because Thou art so good, and I love Thee!"
Imperfect Contrition
Our contrition is imperfect when we are sorry for our sins because they are hateful in themselves or because we fear God's punishment.
1. Imperfect contrition is called attrition. The fear of hell is a common motive of attrition. It is a good motive, but it is imperfect, because it arises from fear of God's punishments, and not from pure love for Him.
A mother sent her three young sons to take a big jar of honey to their grandmother. On the way the boys stopped to play. They stumbled over the jar, breaking it and spilling the honey. They all began to seep.
The first said, "Mother will surely spank us?" The second cried, "She will be so displeased she will give us no cookies!" And the third wept, "Mother will surely be sad!"
The first two boys had attrition: one had the fear of punishment, and the second had sorrow at the loss of reward. The third child had perfect contrition, for he thought only of the sadness and offense he caused to one he loved.
2. To receive the sacrament of Penance worthily, imperfect contrition is sufficient. However, an act of attrition cannot obtain forgiveness of mortal sin without the absolution of a priest.
Even if we feel only attrition for our sins, we can easily develop it into perfect contrition by remembering what we should be without God. We should always try to have perfect contrition in the sacrament of Penance.
3. A purely servile fear of God is not sufficient for imperfect contrition. That is one which makes a person avoid sin only because of punishment: so that, if there were no punishment, he would not be sorry, but ready and resolved to sin, regardless of the laws of God. To receive the sacrament of Penance worthily, purely servile fear would not be sufficient.
We call this fear "servile" because it is the fear of slaves, afraid of a hard taskmaster; they would quickly disobey his commands were they not afraid of his whips. Shall we look upon God thus? Servile fear does not make the sinner turn away from his sin. The "fear of God" that produces attrition is called filial fear. It is a fear of God's punishments that makes the sinner turn away from sin and return sincerely to God; it is the fear that a good son who has offended his father seriously feels when he begs forgiveness.
Regaining Grace
A person in mortal sin can regain the state of grace before receiving the sacrament of Penance, by making an act of perfect contrition, with the sincere purpose of going to confession.
1. An act of perfect contrition takes away sin immediately. Our sins however grievous are forgiven before we confess them, although the obligation to confess as soon as we can remains.
Thus, if one makes an act of perfect contrition after having committed a mortal sin, and then dies before being able to go to confession, he is saved from hell by the act he made. Let us remember the penitent thief: "And he said to Jesus, 'Lord, remember me when thou comest into thy kingdom.' And Jesus said to him, 'Amen I say to thee, this day thou shalt be with me in paradise'" (Luke 23: 40-43).
2. If we have the misfortune to commit a mortal sin, we should ask God's pardon and grace at once, make an act of perfect contrition, and go to confession as soon as we can.
With the act of perfect contrition must be our intention to go to confession as soon as we can. If we die without being able to confess to a priest we shall be saved from hell by our act of perfect contrition.
We may not receive Holy Communion after committing a mortal sin, if we merely make an act of perfect contrition; one who has sinned grievously must go to confession before receiving Holy Communion.
I hope this helps.