My Boyfriend and His Manager...

So my boyfriend and I had an awful break up about 3 months ago, we have since gotten back together about a week later and our relationship I feel has become a lot stronger. Although... when we were on our way to a park to just walk around and talk about us and work through what had happened he was really quiet in the car... like he was thinking really hard about something. So when we got out I asked if he was okay and he said he needed to tell me something but he knew I was going to be mad. Well... it turned out he slept with his Manager from his work... I was very hurt and very angry I know we were broken up but 3 days? A lot was said during that week... he said he never loved me like I loved him, ect. Just a lot of hurtful things. I am moving past this but what I am having issues with is him still being friends with her. He tried to keep in contact with her but that quickly stopped I told him he is going to have zero communication outside of work or we were done. He kept his promise but I see her sometimes because he works with her and they have mutual friends though work but I blatantly ignore her, he tried defending her saying she was nice and he was the only one she slept with from their work, note she is 7 years older and we quickly found out he was one of many and when he let her know that I knew she denied ever sleeping with him. So, I can honestly say that the only reason I haven't said anything to her is because I was asked not to by my boyfriend. But I was forced to see her on Halloween and she tried saying happy Halloween and I just stared at her trying to keep my cool. I legitimately hate her and I feel bad for it because I don't like being this way I'm not like this... but thinking of her being with him makes me sick and I get angry the instant I see her... I would like some feed back cause I need help... I'm going to be seeing her next week because I refuse to let him be around her if I'm not... am I in the wrong here for how I feel?