To forgive is to see something wrong as only a mistake. If an error has been made, even if it is not fixed and there is no apology at all, forgiveness is always the sane and correct response. This doesn't mean you have to go along with a person or situation that you are not inspired to go along with, but the "sin" is not an offense against you to be taken personally.
So what exactly is sin? Sin is where love is not. Anything done in perfect love cannot be an error because love is perfectly honest, open, sensitive, eternally loyal, understanding, giving, and accepting. Within the thought system of love, it is impossible to sin. Sin is only possible when you forget about love as your core value and faith; when you follow the external world's insane lessons instead.
Because so few people have learned unconditional love, the world is filled with sin. The world teaches us that to easily forgive is to let someone off the hook and condone a sinful behavior. But what is the alternative to forgiveness? Non-forgiveness? Non-forgiveness means condemning a person, punishing, attacking in some way, or withdrawing love. Does it make sense to punish someone who is acting without love by withdrawing love?
When there is a genuine willingness, it is entirely possible to correct error. But correction cannot happen if there is attack or punishment because non-forgiveness insanely teaches that the real problem (withdrawal of love) is the solution. If there is non-forgiveness, the problem cannot be solved because the conditions of lovelessness are perpetuated and the ground remains fertile for terrible, disgusting behavior.
This isn't about holding someone guilty, labeling a person as bad, and then speaking the language of forgiveness when it isn't in your heart. If there is any chance of correcting the error of sin, we need to learn to see past the error not as who a person is; rather, a simple, correctable error a person has made. Anything less than total forgiveness is an attempt to make error so real that it is an identity. How can a person change when you have made their error their identity?
By the way, learning the art of forgiveness means starting with yourself. If there is anything you feel guilty of or ashamed of, if there is anything you have not forgiven yourself for, you can now give yourself permission to do so. You can love yourself unconditionally, creating the space in your heart to easily make things right, tell the truth, and to walk the bravest and truest path of all: love.