I think my relationship with dad is forever ruined, what do I do?

For context, I am an only child. My parents have been my whole world all my life. They too love me as much as any parent could. Here's the thing though- my father has repeatedly cheated on my mom. Twice when I was a kid. Once, we suspect, when I had just moved out. And the last time, we caught him was last year - on the same day my mother got detected with stage 4 cancer.

Safe to say, my father has hurt us both a lot. As a child, I've seen a very very toxic relationship between them - where my parents have repeatedly blamed each other for their problems and of course the cheating. My mother never left him, but she obviously couldn't get over it either.

Each time, we did try to forget it and move on. But last year, when the cancer happened and when we caught him, something snapped in me. All my love for him disappeared it seemed. I just couldn't tolerate him anymore. I hated what he did to us and how he hurt us. We didn't move out - because my mother was just starting a very complicated cancer treatment and she needed to be in the home she's been in for 36 years.

I warned him - that he needed to change his ways and prove his worth if he wanted us in his lives. He completely broke from guilt back then too. And this time, I had a lot more power and authority because I am completely independent and married and I can take care of my mom. In fact, I'm her primary caregiver.

It's been 11 months since then. And while I don't think he's cheating - I do see him going back to his old ways like being rude to my mom over small things, creating issues about her family and more. He's a very complicated fellow and he's someone who'll never actually try to rectify his ways. Even now, he's not really involved with her treatment. He goes to the hospital. Keeps updated on her scans etc. But he's not involved unless I tell him. I don't know if he's just incapable of caring or if he's in denial. And now, the whole treatment aspect is really lonely. I just wish he could take some responsibility and actually support me as I become absolutely exhausted of managing everything alone.

But now, I'm just like existing with him in the same home. My parents' relationship is friendly now, like roommates. I, on the other hand, just cannot forgive and forget and I hold onto every bit od resentment and I can't let it go. I talk to him only when required and get into frequent fights with him and am not afraid to call him out. My mother would rather have peace, but I'm not afraid to point fingers and put him in his place.

Imo, he should be worshipping the ground my mom walks on.

I don't know what to do. I used to be so close to him. Now, I barely want to do anything as a family. I get pissed when he does something nice for me because it feels pointless. He's the reason our relationship is ruined but I really regret it.

I know I'll regret this when one day he's gone but what do I do now? I absolutely cannot get over this and I cannot just overlook everything that's happened. I cannot just chill with him like I used to, knowing what he did. The wound in my heart is too deep.