The Bible tells us many things about forgiveness:
1. We have a model for forgiveness. We're supposed to forgive in the exact same way as the Father forgave us in Jesus. Simple. Straightforward. No wiggle room. No "Yes, but"s.
2. We don't have to let them to do the same bad thing to us again. We don't ignore simple prudence. If they stole our pencil, we forgive and clean the slate of that offense. And, also, we keep our pencils out of their reach in the future.
3. It's a matter of accounting, not a matter of feeling. We may not feel like forgiving someone for not paying money they owe us, but we can always choose to tell them that whether we feel like it or not. Pretty sure Jesus didn't "feel like it" when he hung on the cross and said, "Father forgive them, they don't know what they're doing."
4. Until we forgive, God keeps us in jail! Forgiveness lets God deal with anything that needs to be dealt with. The quicker we forgive, the quicker God can intervene.