Since, according to Scripture, our Lord entrusted the power to forgive or retain sins to His Apostles and their followers, why not follow Chris's instructions? How do you know if your sins are forgiven?"
I'll repeat a question I asked earlier.
If Christ intended that human beings ask for forgiveness (and receive forgiveness) through the agency of ordained human beings, and not directly from God Himself, why does the Lord's Prayer, which is addressed to "Our Father", include a petition to have one's "trespasses" or "debts" forgiven?
Wouldn't it have made more sense to have Jesus, when He was responding to a request from His followers to show them how to pray, not include a direct petition to God to have their trespasses forgiven?
This is no ordinary petition to God asking for His mercy. We not only ask Him to be merciful to us, but we dare place a condition and a norm on God’s forgiveness. It is the practice and the measure of our forgiveness to others. We pray for the strength to forgive so that we might be forgiven in return. By offending God in sin, we incur a debt of love and punishment for our misdeeds. We owe God greater love than we would have had we not sinned, and we owe Him a debt of suffering as punishment for offending the Divine Majesty.
So, how do you know when you have sinned? What is the measure against which you recognize your sins? Are all sins forgivable? More importantly, once you believe you have sinned and ask forgiveness from God, how do you know it has been granted?
Wouldn't it have made more sense to have Jesus, when He was responding to a request from His followers to show them how to pray, not include a direct petition to God to have their trespasses forgiven?
The answer is simple, really. Because, when you go to sacramental confession, you are asking God to forgive your sins, and He is doing so through the priest. You don't need the priest's forgiveness about anything, you need God's. That's why you go. And, since you need God's forgiveness, and you seek it when confessing your sins, it is perfectly appropriate to ask for these things, both ahead of the time and after the fact, when making a sacramental confession.
Not to complicate things here, but there is another point. Strictly speaking, only "mortal sins" must be confessed sacramentally. While it is certainly a good thing to confess "venial sins" in a sacramental confession, this is not absolutely "required." Therefore, one could recite the Lord's Prayer (especially at Mass, where one's venial sins are removed by the Mass itself), combined with contrition for such sins (yes, contrition is required for all sins, even venial ones), and such sins could be forgiven at that point.
"What is the Biblical basis for the distinction in levels of sins?" you ask. The answer can be found in 1 John 5:16-17, where John makes a distinction between "mortal" sins and those which are not "mortal." Non-mortal sins are known in the Catholic Church as "venial sins." Mortal sins destroy the sanctifying grace in one's soul, while venial since merely weaken that grace. The total absence of sanctifying grace in one 's soul at death means, by definition, that the person cannot ever enter into Heaven. Thus, sin that destroys such grace is "mortal" to one's soul, hence the name.
I hope this helps answer your question! :-)