Have you read the booklet by Fr. Lehmkuhl, dear 2Vermont? I read it a few years ago, and can't recall anything wrong in it. It suggests many practical ways in which, with the help of prayer and God's grace, we can achieve perfect contrition for all our past sins, not only as one individual act, but have a deep and lasting sorrow of heart for having offended God, who is infinitely worthy of love, thus strengthening ourselves against future falls into sin.
Some other posters have already mentioned some reasons - attrition or imperfect contrition suffices in the confessional, whereas contrition is necessary outside it. And attrition (supernatural sorrow of heart arising from fear of hell or consideration of the loss of heaven) is much easier than contrition (supernatural sorrow of heart arising from having offended God and purely for having grieved His Infinite Goodness). Contrition, however, is not unattainable provided only we meditate often on the infinite benevolence of God, the immeasurable pain it cost Jesus on the Cross to bear our sins and suffer humiliation for our sake, the infinite love with which He nonetheless suffered, the countless labors of His holy life right from His birth in a poor manger, the insults and blasphemies He patiently bore on the Cross while His hands and feet were pierced etc. We can also meditate on the pains Mary endured with Him, the great pangs of sorrow that pierced the Blessed Mother's Immaculate Heart, that She suffered more than all martyrs have suffered, at the thought and sight of what our sins were doing to Her Son, while She nonetheless offered Him out of love for us to God etc. These thoughts will drive us to true love of God and obtain contrition for our sins.
There can be no true contrition or perfect love of God unless there is included in it the desire to fulfill all of God's commandments, which includes going to confession as soon as that is reasonably possible to us, given the circuмstances. In contrition, we unite our will to God's by love, and desire to do all that He desires. Consequently, someone who excludes this desire or dismisses this obligation can never hope to have true contrition.
Finally, there are some other special graces that are reserved to the actual sacrament, like deeper knowledge and light to discover our past sins, a more abundant grace that provides help and strength for avoiding sin in the future, and even a greater remission of the temporal punishment for our sins, which would otherwise have to be undergone in purgatory. And that brings us to a final reason to avail oneself of confession whenever it becomes possible to us even if we are perfectly contrite, which is this - contrition cannot usually be so great (without much effort and practice, and that is why it is very good in any case to cultivate and make a habit of contrition, as Fr. Lehmkuhl teaches below) as to obtain for us the remission of the entire debt of temporal punishment owed for our sins, which debt we have to pay in purgatory, if it is not remitted in this life. May God help and bless you in this time when the Sacraments are hard for you to come by.
WHAT EFFECTS DOES PERFECT CONTRITION PRODUCE?
Some truly admirable effects! For the sinner, thanks to perfect contrition, he immediately receives forgiveness for each of his faults even before making his confession. Nevertheless, he must make a resolution to confess himself at an opportune time; of course, this resolution is included in perfect contrition. Every time he makes an act of perfect contrition, the pains of hell are immediately remitted, he recovers all his past merits, and he turns from being an enemy of God to being His son by adoption and co-heir to heaven.
For the just man, perfect contrition enlarges and strengthens the state of grace. It erases the venial sins he has detested, and increases in him the true and sound love of God. Here are the marvelous effects of divine mercy in the soul of the Christian owing to perfect contrition.
Anyway, a very Merry Christmas to you, 2Vermont, and to all.