To forgive essentially means "letting go".
In other words a person has decided not to let the issue dwell in their minds or consume their thoughts any longer.
One of the reasons some people have a difficult time with forgiving is they believe if they do it's the same as giving a person a "free pass".
Another reason people have trouble forgiving is they assume it means you have to maintain a relationship with the person you forgive which is not the case.
Once you know the truth about someone it's on you to make the adjustment. "Fool me once shame on you. Fool me twice shame on me!" If someone "forgives a debt" it means they've decided to no longer pursue payment. It does not mean they'll extend you another loan.
Lastly some people believe that "forgiving means forgetting". Anyone in my opinion who chooses to ignore "hurtful lessons" is likely to experience them again. One should always learn from their mistakes whether involves examining their criteria for selecting friends or mates. Ultimately we (choose) our own friends, lovers, and spouse. We have to examine why we chose to put ourselves in a vulnerable position with that individual or put our trust in them.
Some people simply don't want to move on! They believe by harboring toxic feelings and thoughts that they are punishing the other person. However they are imprisoning their own minds.
Forgiveness is a gift you give yourself! It's freeing your mind to concentrate on other matters in your life.
It's a decision to move on with your life!