heh, i almost made this thread myself but you beat me to it
lets take away the word "sin" for a moment, as its probably only going to spark pointless religious banter somewhere down the line.
if you do something wrong - and i mean wrong by your own personal definition of the word - what else are you left to do, but forgive yourself? religious fanatics will tell you to "repent" constantly, for you are always doing wrong somehow or another. to me, repenting is simply a route to personal forgiveness.
yet i struggle. simply forgiving yourself is healthier on your immidiate psyche, but in the long run i feel more damage is done by it. my personal views coincide with the "eye for an eye..." philosophy, so if i screw up, i feel i must be punished in some way, shape or form before i am able to forgive myself.
if you do wrong, and forgive yourself soon after - with no consequences or reprecussions other than your own feeling of failure - you are much more likely to do wrong again, and more frequently than if you had some type of real, solid punishment for yourself.
without the punishment, you will probably feel less and less sadness/regret/ect for doing something wrong. ignorant and lazy people are produced this way.