My boyfriend went to a party..

El
3 months ago my boyfriend went to a house party with people from work (we work together) and didn't tell me because 'I was in bed sleeping'. He says he had 3/4 beers and was completely in control of himself which I believe. But apparently this girl that we work with was really drunk and ran up to him as he was leaving and kissed him (everyone at our work knows we are together) he pushed her off and left. He waited until now to tell me all of this. And to make matters worse when he told me she started texting me saying they 'went further then making out' when it was really her trying to kiss him and getting nowhere. She's still texting me saying I should break up with him because he's 'always' flirting with her. I know she's lying. My boss has known her for 10 years and this is what she does to people, she just ruins relationships for no reason, she's done it to him multiple times. And he told me what my boyfriend said was true. So I trust that. 
What bothers me is that he waited so long to tell me. It just made it so much worse and I'm having a hard time trusting him. We've spent the last few days together just trying to return to normal but I still have this doubt in my head and I don't know how to get rid of it (he really is trying to help me) 
And also I have to see this bitch at work everyday. And I know I need to handle myself professionally but I just want to get back at her for making me feel so shitty and really hurting my once perfect relationship. 
So sorry this is long! I just really need some advice or opinions! 
Thanks girls😘