How can I stop myself re-visiting something that causes arguments?

Its basically something that came up in my relationship 2 years ago when we first started out. And we kind of sorted it but I've never been able to stop going back and bringing it up. It sounds stupid but its to do with porn, and me accusing him of watching it as he used to when i first got with him. We row about it and he gets embarrassed over it, he says he doesn't look at it anymore but I can't seem to let it go/believe him and its tearing us apart really. I know this is something ridiculous, I don't know whats wrong with me :/ I just can't get to grips with it and let it go!! any advice at all would be good.