As a newly confirmed Catholic, I must say that setting the record straight about my past sins has been the most crucial process I've had to go through in order to arrive at a clear conscience.
Thank God for priests and confessors - for all the bad press, they play such a vital role in the life of believers that for the mere fact that their office is indispensable, there will always be somebody called by God to serve in their posts.
The popular Protestant belief that our past sins aren't held against us once we profess Christ as Lord and Savior is fundamentally correct, but for an interesting twist: it is never God who is unwilling or unable to forgive, but we as human beings have such fragile and flayed consciences as a result of our sin-stained lives that unless we openly confess our transgressions to persons of authority and perform penance for them, we'll be unable to forgive ourselves. And in the end, to not be able to forgive oneself is to not be able to receive God's forgiveness.
Hence the importance of confession and penance: for those who truly receive the sacrament, it is nothing less than the commutation of a harsh criminal sentence to a ridiculously light penalty, if not an outright declaration of "not guilty" in the face of overwhelming evidence to the contrary.