The absolute assurance is there following a valid confession (when necessary) and till the faithful commits another sin. At that point venial sin needs to be cleansed by a general absolution at mass, and mortal sin must be confessed, to restore the faithful into the state of grace.
It is important to avoid legalism here and not think of sin like we think of crime. Two processes happen to a Christian with vibrant church life. First, through the strength they gather at mass, and a regiment of penance, fast and prayer, sin is gradually conquered. Holiness is not an abstraction: we are all ordered to it, it is the natural condition of man. some do reach the state of holiness in their lifetimes, whether they are recognized as saints officially or not. As St. Francis (I believe) said, to be a saint is easy, you simply have to want to be one.
At the same time, sensitivity to sin grows as well. New Christians often think of sin as a violation of the Ten Commandments, and they intertpret them narrowly. Since most people do not worship idols, do not swear by God, go to church or at least rest on Sunday, remember their parents on Mother's day and Father's day, do not commit crimes or adultery, they see no sin in themselves. Gradually, sin is recognized in acts of selfishness, impure thoughts, anger, self-indulgence, etc. It is useful to recall that while the Decalogue lists sins by what they hurt, a Christian is asked to think of sin in terms of its origin. Another list, that of Cardinal Sins needs to be kept in mind:
So, on one hand, as one grows spiritually, every sin decreases but new sins are seen by the inward eye. We know that a sin has to be understood as such in order to convict the soul. For example, our culture desensitizes us to lust and a product of modern culture commits a sin of lust not understanding that he is sinning. It is then venial sin for him. But a serious mind would discard the cultural deception and recognize lust as sin. At this stage, he has a new sin to conquer. This is why our sinfulness is in constant flux. The absolute assurance of salvation that comes from the sacraments is rarely experienced, and should not be sought because it desensitizes. While there in a platonic sense, awareness of absolute assurance of salvation easily transitions into presumption, in itself a sin.
The virtue we ask for is hope, -- a sense of trust that as we struggle for holiness the merciful God will forgive us. On the two ends of that virtue are the sin of scruple and the sin of presumption.
Regarding "final perseverance", it can ONLY be a gift. We cannot merit this, but we pray for it, especially in the second half of the "Hail Mary": "Holy Mary, Mother of God, pray for us sinners, now and at the hour of our death. Amen" We address our prayer to divine mercy rather than justice. I suppose the key problem I had was with "absolute" assurance. It is difficult to judge anything as "absolute", especially when we base our religious beliefs on faith - which is, by nature, things not yet seen.
Regards
God bless.