Here are a few thoughts that may be helpful.
1Some people think that forgiveness happens quickly, that we decide to forgive and then it's done. Most people, though, need to forgive because they have been hurt, sometimes very deeply, and it takes a long time to heal. That's one reason why Jesus said to forgive up to seventy times seven.
Suppose someone makes your leg break. For many weeks you struggle with a cast and crutches, and you can't do many things you are used to. Long after the pain you feel in the moment that your leg broke, you feel the pain, inconvenience, disability and expense of a slow healing process. So it is as if each day is a new experience of hurt that you must forgive. You can begin the healing process just by deciding that you are going to forgive, but that decision leads to weeks, months, years or of even decades of healing and spiritual work. Growing in forgiveness is an organic process, like growing a tree. The moment you decide to forgive, a seed is planted, and then it may be many days before the first sprout pushes through the soil, and years before the new tree actually bears fruit. Forgiving others is a skill that gets better with practice, like playing a piano. You can't play a piano just by deciding to play. After deciding, you have to practice for months and years to be able to play beautiful music. So if the sister who hurt you asks if you forgive her, just say, "I'm working on it. I'm trying."
2Another thing that makes forgiveness hard is fear that if we forgive we won't get justice--the other person will get away with it. We think that if we forgive them, they might do it again. Or we think that to make it right with the victim the person who caused the hurt should suffer just as much as their victim did. We imagine that we are protecting ourselves from further hurt by not forgiving the hurt we experienced. To forgive we have to let go of resentment and hatred, but that doesn't mean we have to pretend they didn't hurt us, or act as if it didn't happen.
3We can forgive someone and at the same time challenge them, confront them, or even put them in jail. Forgiveness sometimes opens the door for reconciliation. Other times it may be impossible to reconcile or become friends again, but even when we can't reconcile we should still forgive. Forgiveness involves not wanting to hurt the person back; it is not an excuse for their behavior.
4Sometimes we have a hard time forgiving because the other person doesn't apologize sincerely, or won't acknowledge how much hurt has been done. But you can forgive regardless of the other person's attitude. You can even forgive someone who is dead. It may be impossible to reestablish friendship with the offender, and perhaps you will never reconcile, but still you can forgive, because your own heart can change ever if the other person's does not.
5Ultimately only the Lord can forgive our sins, or change our heart. For us to forgive others, we have to call on the Lord's power to change us. There is a connection between forgiving others and being forgiven by the Lord. If we examine ourselves and notice the hurt we have done to others, we can ask the Lord to forgive us for holding on to resentment, hatred and revenge. When we have recognized our own faults and asked for the Lord's forgiveness, it will be much easier to forgive others.