Indeed, it is often the case that a penitant could not confess every sin. Forms of absolution exist that recognize and absolve sins that could not be remembered due to limitations of mind. A desire to confess them is, of course, still necessary.
On the other hand, only sins committed with the consciousness of mind and with the knowledge and choice of committing a sin need to be confessed. These are "mortal" sins. Sins that are objective wrongs but are committed due to ignorance or honest error are called "venial", those are absolved generally in the course of the Mass. They don't need to be confessed.
The consequence of sin remains after the confession. For example, if a sin involves a crime, the priest will impose restitution, but if the restitution is not possible, the consequence remains even after the penance is complete. A sinner free from unconfessed sin, we believe (without pre-judging the judgement of Christ, goes to Heaven, but he stops in Purgatory on his way, where his saved soul is purified so that he is able to have the Beatific Vision of God.
Feel free to follow up. Good questions.