“I cannot forgive Hitler. To do so would be folly given his evil acts. So, in some cases is forgiving an act of folly?”
Forgiveness is a moral virtue. All moral virtues (such as justice, patience, kindness, and love-in-service-to-others) are good. Therefore, forgiveness is good. Goodness is not folly. Because forgiveness is part of goodness, it follows that forgiveness cannot be folly.
That said, you may not be ready right now to forgive certain people for certain unjust actions. This does not make you a bad person. Forgiveness does not have the same quality as justice. Certain forms of justice are so important that they are encoded into laws of the state: Do not murder, for example. Forgiveness is not codified into law because it is the person’s choice whether or not to forgive a given person for a given unjust action. So, if you do not want to forgive Hitler for the pain he has caused to you (and he can cause pain to those who were born long after World War II), then you need not do so, and remain a good person.