A thought by Raymond Brown in The Message of Hebrews on forgetting sins:
When our sins and iniquities are confessed, God pledges himself to forget them. We are incapable of making up our minds to forget. If we determine to forget anything, it is only likely to impress the fact more deeply on our memories. But God says, ‘I will be merciful toward their iniquities, and I will remember their sins no more’ (8:12). God remembers his mercy and forgets our sins. It is the devil who tries to remind us of both our former sins and our present weakness.