Carolyn Hax: Child loses respect for dad who won’t care for himself

He’s married to and lives with Mom, who lives an incredibly active and healthy lifestyle but insists Dad’s fine and doing the best he can. The kids have tried healthy/fun/tasty alternatives — he says no thanks.
None of this is new, but the effects are really adding up and taking a noticeable toll. It’s so very painful to watch.
How do I wrap my head around the fact that he’s an adult, he makes his own choices, and so freaking out over those choices is just frittering away the time I have left with him? His decisions are whittling away my respect for him, and that is a crummy and jerky way to think about your own dad. Any ideas?
Unmovable Dad: There are three issues here, actually, not just one.
Your dad’s self-destructive personal habits are the obvious issue. Of course you’re upset about that, watching as he refuses even basic health maintenance. You have the right idea, though, in redirecting your attention to the companionship he offers as he can offer it. The persistent wanting of something different is the single greatest obstacle to appreciating what we have.
There’s also the futility of wanting this particular different outcome. You’ve all clearly tried. So when the impulse to keep cajoling him overtakes you, recall how you like it when someone eyes your plate and says, “You’re eating that?!” (Shuts things right down for me, at least.)
The second issue is your diminishing respect for him. Yes, it is a “jerky way to think about your own dad,” or anyone. Judging is the mental equivalent of picking scabs: gross, wrong, so satisfying. But … there is a but. You are allowed to have an opinion of your dad as a person, just as you are allowed to have an opinion of anyone else. And that opinion can include updates to the mental list of things you do and don’t love and respect about him, based on this new information. Our opinions can be complex.
So you can love him but not like him, enjoy him but not respect him, this but not that. Whatever. You can feel angry. Sad. Forgiving. The more comfortable you get with knotty feelings about people, the less weird and frustrating the world will start to seem. If ever so slightly less. Recall as needed: You drive your people nuts, too, sometimes.
Third thing: You don’t need you to be swatting yourself down for having complicated feelings specifically about “your own dad.”
I may be misreading you, and if so, my apologies — but I’m picking up a hint of, “He is my dad and therefore I am supposed to love and admire and heed him.” If this is accurate, then the most freeing move might be to dispense with any residual mythic-parent “supposed to” notions, consciously and for good. You are an adult. He is a regular human being. As human as your neighbor, your colleague, the guy behind you on the bus. Bring no expectations, except maybe to learn from this. See if that lifts some of the weight.