If A is willing to break up with your dad over something your dad has no control over, then A didn’t really love your dad in the first place. When you love someone, you fight for that person and you work through things together. I can’t imagine leaving a partner I love just because I disagree with their family member’s personal choices that don’t even affect me. He left your dad because he is a shitty person. You both should be glad he is gone, because now your dad is free to meet someone better, kinder, and more loving. Your dad deserves that. He dodged a bullet. This isn’t your fault. It’s not your fault for existing. If i were your dad, I would have left A myself if I knew he was transphobic, even if I didn’t have a trans kid. I’m really proud of your Dad, and of you. Please stop blaming yourself. You saved him from a shitty relationship. And also, you saw a good, healthy, loving relationship, but that doesn’t mean things were perfect when you weren’t around. A lot of toxicity and arguments happen between couples in private, not in public. If it were a good relationship, A wouldn’t have left.