Every mind changes over time. Parts of the brain alter as they confront war, stress, love, anger, and happiness.
Scientists show that there, at this point, can be no forecasting of what someone will do. For example, mass murderers often have something different light up in the front part of their brain. Contrarily though, someone having that similarity of brain structure is no guarantee that they will be a mass murderer.
You asked, initially, about "sins" but then clarified that you weren't talking about sins as religion views them. I'm confused as to what these "sins" are that you speak of. If they are minor sins, such as being insulting, I think we all agree that people can grow out of that and these sins can be forgiven. Contrarily, murder is something most in society feel is unforgivable. Given this, I find it interesting that the Amish were forgiving when many children in their shoolhouse were murdered. They forgave the killer -- something most of us would not do.
But psychologists tell us that if we hang onto hatred - hating someone who hurt us - we end up injuring ourselves. So, maybe that is what's behind the Christian doctrine of forgiving...we end up helping ourselves, our psyche.
In that end, if people can forgive the most horrible thing humans can do, I would suggest that you be kind to yourself and forgive yourself, your sins. If you know you are different now and would never do such hurtful things, repent, say you're sorry (if only to yourself). And mean it.
Don't over-philosophize or torture yourself -- Instead, do kind things for others. That is the best solution.
Teri Anne