There is a kind of humility that is really unbelief in disguise. It sounds like contrition. It says, “I know God forgives, but I cannot forgive myself.” Underneath, it is quietly insisting that your assessment of your own case is more accurate than His.
Here is something to try today. When the old memory surfaces, do not argue with it and do not rehearse the reasons you ought to feel guilty. Simply say the sentence Paul wrote, out loud if you can: There is therefore now no condemnation to those who are in Christ Jesus, who don’t walk according to the flesh, but according to the Spirit. Then carry on with whatever you were doing.
You are not trying to manufacture a feeling. You are training yourself to trust a verdict. Feelings catch up slowly, and often they catch up last. You do not have to wait for them before you start living as someone who has been set free.