I will be merciful toward their iniquities, and I will remember their sins no more. God speaks here about the New Covenant and makes a final promise: he no longer thinks about your sins. Not because he is forgetful, but because he has decided so.
This is the foundation for true rest: if God himself no longer has your sins on his radar, why should you keep replaying them? What is done is done.
Today you may live in this rest. Not carelessly, but freed. God looks forward with you, not back at your failures.