My Boyfriend Cheated On Me and Is Calling in An Hour What do I Say?

What do I say to this guy? We've been on an off for years and I am so upset. I could never do this to him. I'm worried that I'll either just hysterically do this and say "Why! Why?!" or be crazy angry and start singing You Must Not Know Bout Me by Beyonce. He hasn't even apologized, it's like he has no idea (or maybe he just doesn't care?) how much I am hurting. Is it worth explaining? What do I even say?