I think you meant to say that you couldn't care less, meaning that you don't care at all. Saying you could care less is ambiguous as it expresses a conditional possibility and not something that is certain or assured. Saying you could care less also doesn't give any indication as to whether your current level of caring is high, medium, or low, but only that it's not at zero.
Yep, you are right. Couldn’t is the word I was meaning.