"If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing a deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly." (1 Jn 5:16-17).
Mortal sin kills the life of grace in us; it is deadly to the soul. A sin must meet three criteria to be considered mortal or serious enough to bring damnation:
1) the sin must be a grave matter
2) one must have full knowledge of it and its severity
3) and deliberately consent to it
'We should all realize that no matter where or how a man dies, if he is in the state of mortal sin and does not repent, when he could have done so and did not, the Devil tears his soul from his body with such anguish and distress that only a person who has experienced it can appreciate it.' --St. Francis of Assisi
For the Catholic, mortal sin requires the sacrament of Reconciliation to be restored to God's friendship. One should also refrain from reception of Holy Communion in a state of mortal sin, lest they "eat and drink condemnation on themselves." (1 Cor 11:29).
Now, this is serious business, but we have to balance it with not falling into extreme anxiety or scrupulosity. "Let the wicked leave their way of life and change their way of thinking. Let them turn to the Lord, our God; he is merciful and quick to forgive." (Is 55:7). But it also means not keeping a mental and spiritual 'status check' of our souls, for venial sins can be like the little bytes of data that slow down a computer, accumulating over time and darkening the soul, weakening it and opening the door to mortal sin.
Sometimes it's just good to get back to basics. Lent is primarily a season of repentance and penance. What is repentance and penance? Let's not make it too complicated. Repentance is sorrow for our sins. Penance is our efforts to make amends for our wrongdoing and to repair its effects, while realizing it is Christ alone who forgives us and there is nothing we can do to earn or deserve heaven.
Regular confession helps us to live in the life of grace. Remember....priests have heard it all before, there is nothing you can tell them that is new. They can never, ever, under pain of excommunication, reveal what is told in the confessional, and the priest acts in persona christi ("in the person of Christ") so it is truly Christ who hears and forgives us through the authority given to the priest by Christ himself "to loose what is bound."
So this Lent, do yourself a favor: Make an examination of conscience, using the Ten Commandments as a guide for this "spiritual self-inventory". Go to Confession and be blessed with God's mercy. There is nothing you have done that is beyond his mercy. Do penance to restore what has been damaged. And rejoice that your name is written in heaven with the elect, for there is more rejoicing in heaven over one sinner who repents than 99 righteous. (Lk 15:7)
"He is merciful and quick to forgive."