Nothing is unforgivable but that does not make it easy to forgive some things. It can be incredibly difficult. Sometimes the hardest person to forgive is yourself.
God forgives everything. He forgives but does not forget, but neither does he hold it against us.
Forgiving someone for what they have done, or you perceive to be undeserved, to you or someone you care about, is easier than forgiving yourself. And, forgiving what is done to you is often easier than forgiving for what is done to someone you care about. It seems to me that it is easier to offer forgiveness than to ask for it, even if you are the guilty party.
The Bible counsels forgiveness in various passages but nowhere does it say we need to forget. The most likely origin of the phrase “forgive and forget” seems to be by Miguel de Cervantes “Let us forget and forgive injuries.” writing about Don Quixote. Of course forgetting is virtually impossibe.
Things always remain in our minds, especially what we think of as bad or unjust things done to us. They might become harder to recall with time but they never go away. Memory of injustice to us usually seems easier to recall than enjoyable, or neutral memories.
Not forgiving has consequences, and not just for the unforgiven but for the person who refuses to forgive too. It poisons the mind of the person who does not forgive. It prevents them from moving on as much, maybe more than the person who perpetrated the act for which forgiveness needs to be given.
Forgiving doesn’t mean there should not be consequences for a perpetrator. Consequences in law can be community service, a fine or jail. Consequences personally can last longer, and feel worse than any legal consequences to an action.
Ultimately, failure to forgive will often be to a greater detriment to the person who needs to forgive, than for the person who carries out an act for which they are sorry.