Repentance as a general rule in life doesn't mean to go back and re-repent for offenses you've already been forgiven of. If you'd allow me an analogy here; Take for instance a relationship between a husband and a wife... If the husband gets angry, huffs and puffs, uses name calling in a fight and then storms out of the house, shouldn't he apologize and she forgive? Of course. What about 3 months later when he does it again...does his asking her forgiveness for it over the last incident "save" him from the responsibility he should feel to apologize again for his disruptive behavior? Of course not. When we are "saved" it doesn't prevent us from sinning in the future, of course we have to go back and live in an attitude of perpetual repentance. The apostle Paul said "I die daily". He lived in an attitude of repentance before God, as should we all.

The Atonement of Jesus Christ and the grace He offers us is perhaps the most important thing that we could ever know. It is the core of the Gospel, of our religion, and should be at the core of our lives. We must likewise know how He offers us the riches of his grace, and what we must do to accept and partake of that grace. And therein is the confusion that clouds much of the popular theology of today, a confusion that sadly clouds the thinking of some Latter-day Saints as well.  
The first misconception, which in ignorance has even been taught by a few well-meaning Latter-day Saints, is that we earn our way to heaven by keeping commandments. That is absolutely false. It is completely inconsistent with the scriptures and with all that is taught by the Church. Without the grace of Christ, nothing we did in this life would have any eternal value. All the obedience in the world cannot cleanse one of sin or conquer death - only the grace of Christ gives us such hope.  
The second misconception, one that has become surprisingly common among Christian churches, is that we are saved by grace without any effort on our part, that once we are saved we are guaranteed salvation without the need to keep the commandments of Christ. Now the meaning of this phrase, "keep the commandments," is that we strive to obey and follow Christ, but not that we must fulfill every letter perfectly, for none can do that. But Christ asks us to follow Him with all our heart, and we do that by striving to obey and honor Him. Our failure to be perfect is made up by His grace, for His grace is sufficient to cover our failings and imperfections if we truly strive to accept Him.