What can I do if I continue feeling shame from time to time for my past sin?
"There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit." Romans 8:1
We have been made holy through the sacrifice of the body of Jesus Christ once and for all (Hebrews10:10).
Guilt is a very common factor amongst Christians who have found new life in Christ. Shame and guilt are the weapons that Devil uses to wound Christians in this ongoing spiritual warfare. Satan doesn't want us to believe that God forgives our sins (Psalm 32:5) and purifies us from all unrighteousness (I John 1:9).
In personal experience, when Satan brings up something from my past, I convey this message to him : "Thank you Satan. Thank you for reminding me of what a horrible lost sinner I used to be. Thank you for reminding me just how badly I needed salvation.
Thank you for reminding me about how wonderful the grace of God truly is. I
always appreciate being reminded what God says in Scripture, that while we were
yet sinners Christ died for us (Romans 5:8)."
I don't have to use this approach often because Satan has given up this tactic. Though he still tries to get me down, but when he gets this same reply he doesn't try very hard.
Remember this, when Satan shows you your 'past', do not hesitate to show him his 'future'.
When God doesn't condemn us (Romans 8:1), why should we condemn ourselves (I John 3:20)?