Fairly bad advice.  The truth is actually just the opposite.

God forgets our sins.   Once our sins are forgiven in Sacramental Confession...we should form - with God's help - a purpose of amendment (a decision of the will) to not do x, y, or z again, asking for God's grace in this matter. 

Then we should forget the sin.     Recalling, returning to these sins is very bad for us.  Creates baggage, guilt, distraction = all of which is "distance from God".   

These things take up room in our mind..and create distance from the intimacy with God that HE GREATLY DESIRES. 

And as regards sins and offenses against us when we don't forget we tend to turn into resentments.

A great priest I know told me once to ask the Holy Spirit at Communion time to help me forget some injury that had had happened to me, and it was a marvelous result.

Get to Sacramental Confession and come out clean as a 5 minute old baby.