It's a standard second conditional: would + base form <-> if + past simple

I see this as a type 1 conditional in the past being reported with the usual verb backshifting. At the moment: He says: "Must we pursue this horrible trend of thought?" I think: I am sort of afraid that he will get up and leave on me, if I don't shut up. In the retelling: "Must we pursue this horrible trend of thought?" I didn't say anything. I was sort of afraid he'd [he would] get up and leave on me if I didn't shut up. I am afraid => I was afraid he will get up => he would get up I don't shut up => I didn't shut up