For I will be merciful to their unrighteousness, and their sins and their lawless deeds I will remember no more.
It's never easy to forgive someone who has caused us pain, who have injured us, we have abused us, or have forced themselves on us in some way. But we are commanded by the Lord to forgive others just as Jesus forgives us of our sins. The Bible says that if we don't forgive others, then we won't be forgiven (Mat. 6:15).
The person we need to forgive might not deserve it but we still have to do it. Forgiving someone is a conscious effort of your will. It is something that you have to decide on your own to do. It is not predicated by the person wanting forgiveness, deserving forgiveness, asking you to forgive them, or for them even being sorry for what they did to you.
Forgiveness doesn't mean that you just say, "I forgive you," because you can say it but it doesn't make it true. It's something that you have to will yourself to do but the essence of forgiveness is that after you have decided to forgive someone, you can no longer hold it against them.
You can't bring it up to them ever again. You can't remind them of what they did to you. You can't keep reliving it to make them feel guilty for what they did to you. You don't forgive for them, you forgive for YOU.
You have to let go of the bitterness, anger, disappointment, and yes, even the guilt that you may feel that somehow you were responsible for the abuse in some remote way, even though you're not. You may have been told that it was your fault but you do not make other people hurt you. You cannot make another person abuse you. You can stick around and let them continue to do it but you never MAKE them do it, that's an act of their will.
Even so, we are commanded to forgive and we are commanded to let it go. I'm not saying that it's easy because it's not. It can take years to come to a place of forgiveness for the person(s) who have hurt you in your past. But you have to actively seek that ability through prayer, through the Bible, & through fellowship with people who have been through the same things you have.
Forgiving and forgetting the horrors of our past is how we move forward, and are able to live fully the life that God has planned for us.