How does one purify oneself or one's business from what was unlawful in the past without knowing it, such as financing a commercial shop with an interest-based loan that is still being repaid, or failing to pay debts due to bankruptcy after making an effort to repay them?

You must repent to Allah Almighty from every interest-based loan, because the Prophet (peace be upon him) cursed in usury the receiver, the giver, the one who consumes it, and the one who feeds it to others, as stated in the two authentic hadiths. A Muslim must inquire about the ruling on a matter before undertaking it. As for interest-based loans, you are only obliged to return the principal amount you borrowed. If you were forced to pay the increase (interest), we hope that Allah will forgive you if your repentance is sincere. You should continue your trade and give charity as much as you can to purify yourself and your wealth.