How can one repent from sins of the tongue like backbiting, especially when it's not possible to seek forgiveness from those who were wronged?

There are three conditions for repentance: abstaining from the sin, regretting it, and resolving not to return to it. A fourth condition is added if the sin involves the rights of others: returning what was wrongfully taken to its owners or seeking their forgiveness, as the Prophet, peace and blessings be upon him, said: "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today before there will be no dinar or dirham..." If it is impossible to return what was wrongfully taken because the wronged person has died, or cannot be met, or if there is a fear of a greater harm, then one must increase in seeking forgiveness and praying for them, and praising them, while the penitent increases in righteous deeds.