As a reminder, for sin to be mortal it must be concerning a grave matter which a person commits with full knowledge and consent. The consequence of dying in a state of mortal sin is horrific. For at the moment of death, in the particular judgment, there is no juridical process, no time to make a case for oneself. One is revealed to be either in His grace or not. Mortal sin places one in the "not" category which can only mean a destiny of eternal damnation.
There seems to be two common extreme responses to this reality, and most Catholics find themselves somewhere in the middle. The first extreme is scrupulosity. The individual engages in a near non-stop examination of conscience and begins to see sin in nearly every activity of daily life. Fear of Hell binds this person to a life accented by near relentless guilt. Enjoyment of the blessings of God take on an evanescent quality replaced by a haunting mantra of the soul that whispers, "I'm not worthy." Perfection becomes the order of the day, but such a goal gets frustrated by one's own embedded sinful nature. Analysis paralysis sets in as the person enslaved to his own worries never stops gauging the degree of seriousness of his sin. His code of conduct is supposed to be inviolable yet the flesh and mind continue to betray this code. Christ the savior becomes Christ the antidote to the poison of his fallen nature, and the confessional becomes a sort of urgent care clinic for the soul. This is not a happy man.
At the other end of the spectrum one finds the more Pharisaic response. This person knows as fact which sins are mortal and which are venial, and he will gladly point them out and even offer advice to the less informed. Everything gets done by the book and there are no shades of gray. Salvation becomes formulaic, and the confessional is simply a variable in the equation of attaining justification for Heaven. Christ the savior becomes Christ the ingredient.
Both of these extremes tend to trivialize sin because they each put the human intellect into the driver's seat for determining any given sin's mortality. Each example puts God in the position of jury and the person in the role of judge instead of a relationship of love in communion with the Trinity. Christ becomes more of a score keeper than a conduit of Divine Mercy.
Jesus claims one in baptism as His own. Like a shepherd who loses a lamb, He will call to that soul at the moment of death. A person who truly was out of His grace would already possess the will and desire not to respond to Him. One would have to be so inwardly focussed that the very thought of losing any part of self would make this proposition by Christ unpalatable. This is the ultimate effect of mortal sin; a free will rejection of God; a shielding of the eyes and ears of the soul to seeing and hearing God's will in favor of self interest.
To pontificate that a single transgression punches a mystical ticket to Hell is to play God. Sin is in the will. The action is a physical manifestation of the will. So if one is committing what would be considered mortal sins, then one needs to address the defect that has blinded one to God's will. For if one is blinded in this life, where the body is mortal but the soul eternal, what leads one to believe that the beatific vision will be any less trammeled at the moment of death? It gives one pause to realize that one's position in the afterlife likely will not be a surprise but rather a continuation of what was began in this life.
There is a third response which perhaps proves the most mortal of all. That is the one of indifference or institutional rebellion. This person knows that the Church teaches his action is grave, he may even know why, and yet he commits the sin anyway because he just doesn't care or because it is too easy to continue his action in his way. In America, contraception provides a wonderful example of this. Most Catholic couples in the United States use artificial contraception. Most know very well that it is against Church teaching. Most discard the Church and opt for convenience. They ignore the reality that Church and Christ cannot be separated. They are one. These couples exercise Church out of Christ and believe that their rebellion is simply against an institution they perceive to be out of touch with today.
It would indeed be sinful to make a statement that these errant couples are doomed. God's mercy is unimaginably generous, but it is worth pondering the idea that one's soul does not magically change upon the death of the body. While the soul detaches from the body, its character does not change. If the soul has rejected Christ and His Church in the here and now, is there any reason to believe he will embrace it in the hereafter? If the light of truth is too painful to look at in the now, would it not be infinitely more painful to gaze upon when revealed in its full glory? Ponder that question, but do not dwell upon it for too long lest scrupulosity creep in. It simply serves as a reminder that our hearts and will must always be oriented towards Our Lord, and when one discovers the contrary, that is where the work must begin. It's also prudent to remember that God's mercy extends beyond parish membership. By ways known only to Him, Catholics believe that others who seek God will be brought home to Him. The Church is catholic, that is, universal.
Through all of this, Satan has one hope, and that is that the soul loses hope. For the devil, one of the greatest things is to run across a soul who has forgotten that God is on his side. That is something he can work with. Christ wants one in Heaven with Him, and the great hope is that no matter how far from Him we are at death, if we are at the very least facing God, His mercy will close the gap and bring us into His glory. The Church and her sacraments, especially Eucharist and reconciliation are the means we have to maintain our orientation in the right direction.