When a person who has repented of their sin(s) but continually feels guilty day after day, even after being told that their past has been washed away and their sins have been forgiven, is facing what they call a stronghold. A stronghold is a lie that is believed, which results in an incorrect thinking pattern. The stronghold of guilt is often not alone, it is usually accompanied with an incorrect perception of themselves or an incorrect perception of God (which are both strongholds in themselves). People who have a stronghold of guilt rarely see God for who He really is (including His awesome forgiving nature) or they don’t see themselves correctly. They are new creations in Christ who’s past has been washed away (2 Corinthians 5:17), but they simply don’t believe it because they have a stronghold in their mind that needs to be torn down.
If this describes you, then I highly recommend reading the teaching on Strongholds.