Is atonement for past sins and mistakes possible?
The answer depends on what kinds of mistakes you have made. However, in general, mistakes can be atoned while sins can only be forgiven at best. mistakes that we make in daily basis are not intended to harm someone or performed under evil will. It may be made by careless thinking or lead by emotions. Therefore, they can be reconciled by compensating if damaged or harmed anyone or anything, or by learning not to make the same mistake again later on. Once someone said that mistakes and failures are the mother of success. I think it is absolutely true.
On the other hand, sins have different meaning than mistakes. Sins usually followed by evil will and performed on planned basis. I don't think it will be reconciled even they compensate it. It can be forgiven at best yet it will never be forgotten.