Forgiveness is 100% an inside job. The sooner you take responsibility the sooner you heal. Since it is you who were hurt, and it is you who must heal, you take ownership of accepting the offense. Taking ownership removes the likelihood that the offense will control your thoughts and actions. It reminds you that YOU control your thoughts and actions, always have. You don't have to go from day-to-day reliving past painful experiences.
Learn that forgiveness is necessary because you deserve freedom. Acknowledge that you played a part and that in time you will heal from it all. Accept the lesson you were meant to learn from the experience.
You will NEVER need an apology for closure if you remember the following realizations:
Realization #1: Forgiveness is necessary. If you don't forgive an offender your life remains looped, stuck, and held hostage by the pain
Realization #2: Freedom comes after forgiveness. Letting go can be a process. But the process of letting go is more valuable than "understanding" why someone hurt you. Understanding only justifies. Letting go frees you from the pain. Do the work and learn to LET IT GO, for good
Realization #3: Forgiving yourself reminds you that you're human. Most importantly, make sure you acknowledge the part you played. It's cliche, but WE REALLY DO ALL make mistakes. The acknowledgement removes the sting a bit and helps you to learn through the process
Realization #4: The pain subsides over time. It has been said that time heals all wounds. This is fact. This too shall pass and eventually you will experience forgiveness personified, which is the peace that comes from acceptance. You will heal, life will go on, and you will come out much better than before. You really will
Lastly, none of the aforementioned will be easy. Nope. Not in the least. However you will find that if you work the inside job of forgiveness you will feel lighter. You will remove the heavy elephant off your back and release the extra baggage you may be carrying.