I think this question and answer underscores how our culture is just entirely unequipped to deal with grief. There is literally no good way to have this conversation, only various levels of awkwardness. LW doesn't want to be a downer! She doesn't want to appear callous! She doesn't know how to respond to sympathy! LW, your father is literally dying, it isn't your job to smooth things over or make people who are not you feel okay. Be honest, accept the sympathy. "I've made my peace with it" is a totally normal response when you have a parent suffering from a slow terminal illness.

Other things you can say:

Thank you. [change of subject]

I appreciate it. How is [important person or thing]?

*sad smile, acknowledging nod*

It's hard. Can we talk about something else?

You are absolutely entitled to pay as much attention to their response as you feel up to before changing the subject, which will also let people know that you DON'T feel up to discussing it further and you WANT to be distracted by happier things. Right there, in that moment, YOU are the center of the "support in, dump out" concentric rings, and you get to decide what kind of support you want to get and you DON'T have to worry about what they will do with the unpleasant news. They get to take it to the next ring out and get their own support, if they need it.

It sucks and I'm sorry you're going through this.