WOULD YOU TRUST YOUR BOYFRIEND OF FIVE YEARS IF HE DID THIS?

My boyfriend and I have been together since 2006. He started a part time job in 2008 after talking to a girl at his gym who set him up at a mall job that sells sunglasses. In 2009 we broke up for a month. After we got back together this girl was texting him about work calling him "babe" . I was suspicious that he got with her when we were broken up but I let it go (he made a strong attempt at getting back together so I figured if he had feelings for her it didn't matter.. he wanted me). We have been together ever since. On New years eve he had gotten drunk. His mom kept texting him asking if he was ok because it was 5 am. I finally answered to put her at ease but noticed there were several texts to this same girl... starting early that day. The last one said "Nikki.... you're the sh*t! I love you city girl" .(I guess because she works in the Soho NY store as well as with him in NJ). He did this at 11:55 pm on New Years eve then ran down a minute before midnight to join me.. I was hanging out with all of his friends. She never answered the last text, but I feel he was obviously flirting with a girl he had feelings for (my strong guess). He didn't text ANYONE else and we both have friends of the opposite sex that I know are really just good friends. He apologized the next day and says he was drunk (but he text her was before he started drinking) and when I bring it up he says "get over it" now. I can't. I am thinking of ending it because my feelings for him and trust has changed. opinions please!!