Forgiveness is not for the offender. Forgiveness is for the "victim".
It's for our health that we forgive. It's for our peace that we forgive. The definition is simple, "To give up the WILL to punish".
Spare the rod and spoil the child. This is true. But, people often don't understand discipline and equate discipline with punishment. We can see this in the prisons. We want the wrong doers to be punished. Punishment leads to no good end. (although there are theories of learning through pain association, but that's a whole other topic).
If my kids do something wrong I discipline them. I don't punish them. The fact isn't to make them pay for their mistake. My intentions are to teach them and instruct them in order to help them grow and learn. If my kids do something wrong they may get grounded or scolded. I may be angry, or sad or whatever emotion is naturally involved. I don't hold a grudge against them and keep making them pay for their mistake over and over. After the disciplinary action we go right back to our normal relationship and hold no grudges.
Revenge is something that us humans use to make ourselves feel better. It doesn't teach the offending party to do better. Although a quick punch to the throat might make them leave us alone for a while. It doesn't change what happened. It doesn't teach them anything about what they did. It just makes us feel better at the moment. Then later, we refuse to forgive, and we carry all that stress around with us and it grows and festers in other ways.
So, forgiveness is for us. Go ahead and be selfish! Think about YOU when it comes to forgiveness. Just as you're only thinking about YOU when someone does wrong.