And yes, either way - it non-forgiveness or forgiveness cannot change the past, but it does change the future.
Personally - when I find it hard to forgive and move on - I check out a couple of things.
What can I learn from what happened - the core data - about myself and my relationship with the other person?
I look at my patterns - what I did to find myself in the situation and determine not to do that again - turning to my Manisfesto of guidelines to help me create patterns to bring love, light and energy into my future.
I also decide if I value the relationship enough to step into the 'other's' shoes as much as I can - and this might involve confrontation - to see
if there is an unknown (to me) factor in the situation - I check it out.
Or - I choose non-condemnation - refuse to get hooked and pour my precious energy into a situation that has passed and can't be understood or changed.
Then - if I still find it hard to 'let go' - I check in to see if I can 'let go' of the energy around person through my decision to have love and compassion for all beings, and my mission to create light and love at every opportunity. Stepping up into my Higher Self consciousness - spirit 'beingness' if you like.
Ceremony is another avenue that creates light energy around situations I find myself 'stuck' in. There are many ways to do this and the one I keep finding myself turning to is the most simple of all. Place and object in the ground in front of you, as stone or shell or flower, and surround it with blessings of gratitude for the experience - what you have gained from going through it - and call in what you need to get past it. Ceremony can also be used to surrender what you can't manage over to your Creator, to Mother Earth's heart, or your Ancestors who walked in a way that you would like to walk.
Alternatively, I might choose to leave the energy around the situation there as a reminder not to go down that path again. This is a conscious process - with a daily practice and reminder that holding on to the past can only be productive if it prevents you from going down the same destructive path again - keeping you safe so to speak. Domestic violence and being in relationship with someone with an addiction is typical of what I am talking about here.
At all times - I attempt to follow my Manifesto. When you sort out what your boundaries are - and stick to your Manifesto of rules and principles to feel safe and live a productive, authentic and full life- you find it much easier to 'let go' of the energy around the past - and create more space for wiser actions in the future.
Looking at your experience objectively to learn the lesson well and gain as much wisdom as you can - creates energy for the future. Treating the experience subjectively - taking it all personally as if it is all about you and has nothing to do with the other person's personal circumstance - will keep you trap you in the past story, and prevent you from moving forward to create a new story about you and your life.