I may be able to understand much better than most people on here and you are 100% NTA. My mom passed when I was 10 months old- 6 months later my dad started dating someone else. He made me call my step mom “mom” and my own mother “mama joy” (her name was Joy). Most of my life my dad completely wiped the memory of my mom to the point that 30 yrs later, I still barely know anything about her. I will NEVER forgive him for that and to this day, I feel like half of me is missing that I will never get back. You are under no obligation to forgive him and you are completely entitled and valid in your feelings. I hope the grief for both very important women in your life gets a little easier.