I feel like an episode of Friends

Isabel
A while ago I told my boyfriend I needed a break. I was going through a lot at the time and needed to work things out in my head. It lasted a month before I was ready to pick up where we left. I feel like it's made us a stronger couple. He recently told me that during that time he slept with another girl. And went on a date with a different girl. I had no clue. The one he slept with he even tried to get us to be friends but I didn't like her. I mostly didn't trust her even before I found out. Knowing this is tearing me apart. I stayed loyal to him the whole time. And I feel like he would leave me for her if she tried. I can't even think about it without crying and it makes me want to puke. I've tried explaining to him how much it hurts to know but I don't think he understands.