Asking Forgiveness for Speaking Ill of My Father - Feeling Terrible

Assalamu alaikum. I need some advice and wanted to share what’s been weighing on me. Before I became more committed to my faith, and for a bit after, I used to complain to my then-boyfriend (now husband) about my dad’s behavior. At my brother’s wedding he was drinking, and my partner didn’t like how he acted. To calm things down I told him I had my own issues with my dad - that he gets angry, that I’ve felt hurt by how he’s treated us over the years - basically I vented about him behind his back. There were times when he’d shout at me for no reason and I’d quietly mutter bad words about him (never directly to his face). I realize now how wrong and disrespectful that was. I always knew backbiting is a sin, but I didn’t fully understand how serious it is - that even if I try to be a good Muslim otherwise, this sin could be heavy on the scales if the person I wronged doesn’t forgive me. My dad hasn’t been stable mentally and we’ve had many family conflicts. He doesn’t practice, and yet I can’t bring myself to hate him, even though his actions hurt me. Now he’s been diagnosed with cancer. I feel crushed. I can’t bring myself to apologize in person because I’m afraid it will cause more upset and confusion for him. I’ve bottled this up for so long, and the diagnosis has made the guilt overwhelming. Am I doomed? If my dad doesn’t forgive me on the Day of Judgment, does that mean my repentance is useless? Will Allah forgive me even if my father doesn’t? Is there anything specific I can do - any duas, acts, or ways to ask forgiveness on his behalf? I really need guidance. JazakAllahu khayran.