I think you are on to something about the devil’s wanting us to continue to feel guilt in sins of which we have previously repented. He has reasons for wanting us to doubt God’s love and forgiveness.
Where I get hung up on, is that in some sins in the old testament require the death penalty, murder, adultery, homo-sex, cursing your parents, etc, but in the New Testament we can be forgiven of them. That is one source of my doubt.