Why do we want to hold on to the things that would weigh us down? Carrying around the burden of past hurt does us no good. It robs us of our joy and peace and keeps us stagnant spiritually. Are we just saying we forgive because it is what is expected of us as believers? While every situation is not the same, we still need to live according to what we know to be right in the eyes of the Lord. Forgiving a sinner versus forgiving a brother or sister in Christ may play a role in why forgetting can be difficult. Some people are set in their ways and continue to do us wrong time and time again.
As hard as it may be, we must get it right as believers. To forgive but not forget is half the journey in loving unconditionally. The kind of love the Father has for us which He showed by sending His Son to die for us while we were still in sin. (Romans 5:8)
Jesus went all the way to the cross for us. Victory over sin could not have been accomplished if He had only gone half of the way. We cannot do this by ourselves or in our own strength so we have to prayerfully seek the help of the Lord to be able to do this wholeheartedly.
Hebrews 10:17
And their sins and iniquities will I remember no more.