My parent will not admit that anything ever happened, and they have not changed at all. How am I supposed to forgive that?

“And Saul cast a javelin at him to smite him: whereby Jonathan knew that it was determined of his father to slay David.”

Jonathan finally sees his father clearly at a family meal, when Saul throws a spear at him for defending David. The text says he got up in fierce anger and grieved. He never receives an apology and he never gets a different father, and he still lives a life marked by loyalty and love. Seeing a parent accurately, without the hope that they will become someone else, is its own kind of loss.

“Be ye therefore merciful, as your Father also is merciful.”

Jesus is teaching a crowd that included many people with very little power in their own families or under Rome. His instruction to be merciful is grounded in a comparison rather than a threat: mercy like the Father's. Notice that he does not tie it to the other person's repentance. He is describing a way of being that stays yours to keep even when the other person will not move an inch.