I don't even care
Why don't you care about our discussion?
I'm totally apathetic to our discussion.
Can you explain why to me? I think that statement feels like I am being devalued as a person.
You shouldn't care about anything so it doesn't matter to me.
There are more polite ways to excuse yourself from a conversation if you no longer want to talk. You could also try to change the subject and find something else interesting to talk about.