Have you ever read or listened to Ester Perel's work regarding why people cheat? Might be helpful to explore at some point. This doesn't mean they don't love you. And you don't need to break up with him or her. Maybe you do. But you don't have to jump there so quickly. It WAS a terrible, traumatic betrayal and when the initial crisis settles a bit I would try to understand WHY this happened. Did they think about you while it was happening? Had there been an attraction before? Did they ever talk about it? And also, you need to figure out what really happened. One of them is lying, or both. Probably your boyfriend. Sounds like he was minimizing and trying to place the blame on the friend (not cool, at all).

Take some space and be gentle with yourself. This kind of pain is HARD on you, even physically, and if you can take off from work (it's the holidays, so maybe that more likely) and spend time with other people you are close to, it can help you get through the worst of it.