How can one find deliverance from the depression of the soul and the hardship of life resulting from past sins, while being certain that Allah is Forgiving and Merciful, and [yet facing] the difficulty of redressing moral grievances?

As long as the sins do not involve material rights of human beings and you have repented from them, then repentance erases what came before it. Allah forgives all sins, no matter how numerous they are; in fact, He transforms them into good deeds, as Allah the Almighty says: ("For those, Allah will change their evil deeds into good ones"). What you feel of despair and hopelessness is from Satan, and Allah says: ("Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah.'"), and the repentant person knows that he has become beloved to Allah, as He says: ("Indeed, Allah loves those who repent"). However, if the sins are related to the rights of people, such as backbiting and cursing, then you must seek their forgiveness, unless doing so would lead to greater harm. Otherwise, you must seek forgiveness for them, speak well of them, and increase in acts of obedience.