Help!

I had recently got into a fight with my boyfriend, however it's not the fight that I'm bothered with, it's how do I express that I'm sorry? We had broken up for three weeks, I was upset and looking and searching for something to help me get over being sad... I had sex with another guy. I never contacted this guy or anything afterwards or before. I told my boyfriend and of course he got mad, which he has the right to. But in my defense we weren't dating, point is what should I do?