When my dad died I realised that by complete fluke, I was incredibly lucky not to have any 'unfinished business' with him, iykwim. I miss him, I would love to talk with him again and to show him the dc's, but I'm not haunted by anything I wish I'd said, or apologised for.
A year later, my mum was diagnosed with the same cancer that killed dad. At the time I hadn't spoken to her in 4 years - she is hard, hard work, my mum - but I was the only one of her 4 kids in the country, so of course I went to see her in hospital. I realised that if she died, I'd be full of guilt about our poor relationship, so I'd better work on it.
It's been 11 years now (she recovered from the cancer, though has many other health issues) and I am still working on it. Part of the reason I moved back to Oz from UK was so she could see the dc's, but she doesn't much - partly ill-health, partly her own social life, and partly her own peculiar attitudes. We see much more of the IL's who live the other side of town but always ready and willing to help, listen, talk, etc. And she does make me nuts, a lot of the time.
I wish it could be better but I've realised, she would have to want that too, and work on it. (Perhaps she is; perhaps she can't; perhaps she doesn't know - think this is most likely.) I know I've tried to be as honest with her as I can, once I even said to her that what she was doing wasn't helping our relationship any - she was astonished and then deeply apologetic. She really just hadn't a clue.
If you are happy with how you're behaving, and the efforts you're making, then really that's all you can ask. Your dad, like my mum, is doing the best he can, given his situation and what's in his head. Maybe give him a 'heads up', let him know that you'd like things to be better, or just quietly make an extra effort yourself - then try to accept whatever he can give and make your peace with that.
apologies for the essay, hope it helps a little.