How can one forget past situations of criticism and humiliation that still linger in the mind despite the passage of four years, and is a person sinful for remembering them?

Not forgetting a transgression is not a sin, because thoughts and self-talk are forgiven as long as they do not lead to action, according to the words of the Messenger of Allah, peace and blessings be upon him: "Indeed, Allah has overlooked for my Ummah whatever their souls whisper or talk about, as long as they do not act upon it or speak of it." However, forgetting a transgression indicates a more tolerant, forgiving, and generous soul. What helps with this is remembering the immense reward of forgiveness and pardon, and remembering the Day of Judgment.