What should I do about a ring I found 20 years ago, which I ignorantly disposed of, and now realize my mistake, knowing that I have moved out of the house and my relationships with the neighbors have weakened?

The duty was not to dispose of the ring and to announce it for a year. If despair of reaching its owner becomes likely due to the long duration, then announcement is not obligatory. If you identify the owner of the ring and he agrees to the sale, pay him the price for which you sold it. If he does not agree and it is not possible to return the ring, then its owner is entitled to its value. If it is impossible to ask the neighbors and it is highly likely that the owner of the ring cannot be reached, then you must repent and give charity equal to the value of the ring, guaranteeing it to its owner if he appears. The correct view in the madhhab is that it is permissible to give in charity a found item (luqata) that is not owned through announcement. The intended meaning of "value" is the customary price. The value of the found item is guaranteed on the day its owner is known, or on the day of its disposal, or on the day its replacement is paid. The more cautious approach is to consider the value of the ring on the day of giving charity and to disburse that amount.