How should a Christian deal with feelings of guilt regarding past sins, whether pre- or post-salvation?

Everyone has sinned, and one of the results of sin is guilt. We can be
thankful for guilty feelings because they drive us to seek
forgiveness. The moment a person turns from sin to Jesus Christ in
faith, his sin is forgiven. Repentance is part of the faith that leads
to salvation (Matthew 3:2; 4:17; Acts 3:19).

In Christ, even the most heinous sins are blotted out (see 1
Corinthians 6:9-11 for a list of some unrighteous acts that can be
forgiven). Salvation is by grace, and grace forgives. After a person
is saved, he will still sin, and when he does, God still promises
forgiveness. "But if anybody does sin, we have one who speaks to the
Father in our defense—Jesus Christ, the Righteous One" (1 John 2:1).

Freedom from sin, however, does not always mean freedom from guilty
feelings. Even when our sins are forgiven, we still remember them.
Also, we have a spiritual enemy, called "the accuser of our brothers"
(Revelation 12:10) who relentlessly reminds us of our failures,
faults, and sins. When a Christian experiences feelings of guilt, he
or she should do the following things:

1) Confess all known, previously unconfessed sin. In some cases,
feelings of guilt are appropriate because confession is needed. Many
times, we feel guilty because we are guilty! (See David's description
of guilt and its solution in Psalm 32:3-5.)

2) Ask the Lord to reveal any other sin that may need confessing. Have
the courage to be completely open and honest before the Lord. "Search
me, O God, and know my heart; test me and know my anxious thoughts.
See if there is any offensive way in me, and lead me in the way
everlasting" (Psalm 139:23-24).

3) Trust the promise of God that He will forgive sin and remove guilt,
based on the blood of Christ (1 John 1:9; Psalm 85:2; 86:5; Romans
8:1).

4) On occasions when guilty feelings arise over sins already confessed
and forsaken, reject such feelings as false guilt. The Lord has been
true to His promise to forgive. Read and meditate on Psalm 103:8-12.

5) Ask the Lord to rebuke Satan, your accuser, and ask the Lord to
restore the joy that comes with freedom from guilt (Psalm 51:12).

Psalm 32 is a very profitable study. Although David had sinned
terribly, he found freedom from both sin and guilty feelings. He dealt
with the cause of guilt and the reality of forgiveness. Psalm 51 is
another good passage to investigate. The emphasis here is confession
of sin, as David pleads with God from a heart full of guilt and
sorrow. Restoration and joy are the results.

Finally, if sin has been confessed, repented of, and forgiven, it is
time to move on. Remember that we who have come to Christ have been
made new creatures in Him. "Therefore, if anyone is in Christ, he is a
new creation; the old has gone, the new has come!" (2 Corinthians
5:17). Part of the "old" which has gone is the remembrance of past
sins and the guilt they produced. Sadly, some Christians are prone to
wallowing in memories of their former sinful lives, memories which
should have been dead and buried long ago. This is pointless and runs
counter to the victorious Christian life God wants for us. A wise
saying is "If God has saved you out of a sewer, don't dive back in and
swim around."