Forgiveness is giving up the hope that the past could have been different. That one sentence helped me tremendously in many areas of life. You, at the time, were doing your personal best. So was your father. It didn't work out. Let it be. You aren't some horrible son. Have compassion for yourself as victim of a cult as well.
I'm sorry it didn't work out in the best way, there are people estranged from their families for all kinds of reasons. My dad died in April, spending the last year of his life shunning me after I left the dubs. We never had a great relationship. I tried very hard and he pissed on it all in the end. I took him fishing, to baseball, games, on trips, all of the things I could do to foster a relationship with him because he was never interested in one when I was a kid and I wanted one desperately. In the end he threw it all away. But I also realize that his parents were very messed up and unfortunately hurt people end up hurting other people. I had a lot of issues around all of that. Back to that first sentence though, forgiveness is giving up hope that it could have been different. He was doing the best he could with the tools he had and the damage that lived within him. You were doing your best. Your dad did his. It is unfair to ask for more.