You can leave a conversation without leaving the relationship.

Stepping out of a discussion often gets treated as an ending, by both people. You stop, the other person hears rejection, and so the pressure builds to stay in a conversation long past the point where either of you is thinking clearly. But a pause is a fact about tonight, not a verdict about the relationship, and saying so out loud is usually all the difference requires.

When you next need to leave one, say when you are coming back and then come back. Something like: I want to keep talking about this tomorrow. Then let the room be quiet without making it cold.