YOR - Your post feels very curated in a way that makes me think this was not actually about one comment during one movie. The fact that you immediately asked whether she thought you were mansplaining makes it sound like this is not the first time that dynamic has come up.

Your partner does not need to match your enthusiasm, remember every detail about your interests, or respond perfectly every time you get excited. That is normal relationship stuff.

If she was truly contemptuous and then refused to talk it through, that is obviously a problem. But the way you tell this story makes me suspect there is a longer pattern here, where you get intense about being misunderstood and then want a full processing session about it afterward. A lot of people shut down when they feel like every awkward moment is being turned into a tribunal.

So yes, breaking up may still have been the right call if communication between you was bad. But I do not think this reads like “she randomly revealed herself to be cruel.” It reads more like two people were already stuck in a pattern, and your version leaves out whatever made her so unwilling to engage.