How can one atone for backbiting and oppressing people if he repents and forgets those he wronged, while fearing the loss of his good deeds in the Hereafter?

The conditions for repentance are: abstaining from the sin, regretting having committed it, resolving not to return to it, and restoring injustices to their rightful owners or seeking their forgiveness if they pertain to the rights of human beings. If it is impossible to seek forgiveness from the wronged person due to their death or fear of causing greater harm, then it is preferable to conceal the matter, increase supplications for forgiveness (Istighfar), make Du'a for them, praise them, and perform righteous deeds.