What is the solution for someone who has repented to Allah Almighty yet still feels guilt and depression, believing that what they have done will not be forgiven by Allah?

One must not despair of the mercy of Allah, for Allah forgives all sins for those who repent, transforms their evil deeds into good ones, and rejoices in His servant's repentance.

Furthermore, Allah has warned against despairing and losing hope in His mercy, for despair is a characteristic of disbelievers and those who have gone astray, and Allah's mercy encompasses all things.