Help... He somewhat cheated or did?

My boyfriend of five months somewhat cheated on me with a coworker. He hates her prior to this happening. But she got his Snapchat and they started talking and sexting and then he went to go see her for a hug. This all happened in a span of four days. I found out while I was on his phone closing all the apps. He lied to me four two days and told me that it was a friend. I don’t know what I should do he’s stopped all communication with her. And I want to work it out but I feel stupid and there are times where I’m just so angry. Please help give me advice because right now I’m so scared of my thoughts and my mental state right now.