It is hard to live under the shadow of someone else's unforgiveness if you are letting them be your judge and accuser.
Don't absorb their pain the way they have absorbed from others. Move through it. Yes, I know you feel like can't breathe sometimes. I know it makes you feel pain you didn't know still lingered or didn't want to acknowledge. Psalms 23 says "Though I walk through the shadow of death (and it feels like or may be a death) I will fear no evil, for You are WITH me."
Move through it.
Take it to the Cross.
Even if you take two steps forward and one back- if you hold onto grace you are still making forward momentum.
They are not your judge.
Know your Judge.
God is your judge and He does not give us what we deserve. He is kind and merciful, long suffering and gives grace to the humble. He does resist the proud but it is because of His great love to draw you to Him. It isn't because He desires to be at odds with you. Do you not stop your child from crossing the road when a car is coming to keep them safe? How much more love does God have for you? Just as you see things your children do not, so He sees things you do not. He can be trusted.
They are not your accuser. We do not war against each other but against the accuser of the brethern. But the accuser will try to make it about you vs them. I'm not negating the pain they've caused (or you have caused) but the story behind it doesn't make it who you are.
Know the accuser. Satan. The one who comes to kill, steal and destroy. He will use your wounds to dig in and set up shop if you allow him. He only has residence if you let him. And he is soooo sneaky and cocky to move himself in. He will use what you resort to to medicate your pain (food, alcohol, sex). He will tempt you to fight back your way or run away from what hurts so deeply and give up.
If you are in a relationship with Jesus, the accuser has no right to point a finger at you and belittle, shame, condemn you. He will take any half truth to get you to believe it, twist it and push it in deep into your heart. He will embellish it and stoke the bitter fire that has been kindled. But you are in right standing with God if you've accepted Jesus as your Savior and you are deeply loved. He loved you before you found Him. He had a plan for you before you were conceived. He knit you in your mother's womb. No human can change how He sees you, what you are worth to Him or stop His plan for you. But you have to trust Him.
Have you ever obeyed God only to find yourself in an impossible hopeless position? It can make you doubt if you should trust Him, can trust Him and maybe even a bit bitter for having been placed in that spot.
The Israelites stood at the Red Sea (Exodus 14) because God told them to go there. He told them Pharoah was going to come back for them. Yet they panicked. IF THEY HAD TURNED BACK THEY WOULD HAVE MISSED THE MIRACLE AND PERISHED. You can't ask for a miracle and walk in the opposite direction of where God is calling you to trust Him.
I know it is hard to stand. I know it is hard to wait on Him. The Red Sea took all night to part and that must have felt like a million years. One moment in a place of fear, hoplessness or shame can feel like eons.
You can take responsibility for your mistakes and messy parts in life without letting the accuser make it your identity. I know it would be easier if the other person would forgive you or acknowledge their own part in your mess too but honestly, you can't control that. It isn't your responsibility. What if they never do?
Are you going to miss your calling and all that God wants to birth in you? Put Him first and what He says about who you are. Be grateful for forgiveness and grace and reconciliation with others if it comes- it will be a gift-and if you know it is by God's grace and not your doing, it will be an even sweeter gift. Just don't let who you are in Christ be dependent upon it.