I suggest turning your good mind to romantic relationship theory and communication theory long before breaking up.
Note that nobody, not even you, are infinitely rational or intelligent.
Note also that it's rational to make judgment calls about what to think about and what not to. And it's rational to feel different ways about the same conclusions. One person might believe in AI x-risk and decide to not work on it and not worry about it because they think they'll be happier, and the rest of the world will be almost exactly the same. It sounds like your partner hasn't thoroughly grappled with the issue, so I'm betting her real epistemic state is uncertainty. So her real judgment is to not worry about it to much.
I'm betting that you've argued about it, and that has created important emotional responses in both of you that have distorted the logic.
When you do that research on communication and relationship theory, I think you'll wind up wanting to have another discussion with her on the topic, but being really really careful not to let it turn into an argument that will distort both of your rationality. Which is hard when things are important.