A lot of things in life are deemed "unforgivable".
Breaking a promise. Breaking someone's trust. Breaking someone's heart.
The people that are the closest to you seem to be more prone to hurting you and doing what we may think is unforgivable. When we can't move past this, we become prisoner to our worst experiences.
When this happens to you in your life, and someone you care about disappoints or betrays you, it's easy to become bitter towards the world. God wants us to respond in a different way when we are hurt and full of anger.
Read these words that Jesus told his people:
"Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you."
(Luke 6:27-28)
In the following verses Jesus gives several examples of how to treat those who have hurt you, and He concludes with,
"Be merciful, just as your Father is merciful." (vs. 36)
When you feel hurt, Jesus simply says that we are to love our enemies or the people that have caused you pain. Although this isn't always easy in our lives, it is needed and required from everybody. A person's normal response to someone who hurts them is to hurt them back. When we do this, we fall into the same sinful gutter the other person is already in. We stoop to the person's level, and we ourselves are no better than the person who has hurt us.
Simply pray for the person, give it to God, and go to sleep.
The unforgivable is hard to forget, and it can be hard to pray for a person who has hurt us. This isn't about the person: it's about you. When you hold onto all that anger and hurt and choose not to forgive that person, you are doing more harm to your well being than you are to theirs.
If you don't get past the wounds of the past, you will bring them into everything else you pursue. No one said forgiveness was an easy process. The first step to forgiving is understanding what forgiveness is--it's not reconciling or forgetting. However, it is a decision to let go of resentment and thoughts of revenge for that person, and reaching a place of understanding for that person.
Forgiveness doesn't excuse or justify what that person did; it helps you reach an inner peace. You are not doing this for them, you are doing this for yourself. We all do things that can be unforgivable and we all need to be forgiven.
This all means that the greatest benefit from following God's words to bless those who have hurt and betray you comes to you. God will bless you. Even though you are hurt and betrayed, forgive the unforgivable.