Am I overreacting?

Story: My boyfriend recently admitted to me that he had flirted with one of his coworkers about a year and a half ago. 1. I don't think flirting with other people is acceptable in a relationship if you are serious. And we are. We have been dating for six years. 2. I feel like this is a form of cheating. A minor form but still cheating. 3. I have never liked this girl and I never knew why. I guess now I do. When this happened, we weren't in a great spot. We were always fighting and were very unhappy. (Up until I found this out we were doing great. We had worked through everything). I understand how during that time that I was at fault for many things and have worked hard to try and make things better. When he first admitted this I was extremely hurt and I cried. We tried to work on things and to move on. A few days later though he came back to me to say that it wasn't exactly flirting and that he felt like he didn't do anything wrong. In my eyes, he basically took back everything he just said and created excuses for everything. That hurt me again and of course I cried some more. After a lot of talking, he has finally come out to say that he made a mistake but he still holds his ground that he didn't flirt. He has agreed to stop talking to her. (He called and texted her fairly so often and would talk about personal stuff. We have even gone out with her and other friends but this was before I knew what happened). He has been doing this but the other day I found him liking her personal photos on facebook and I got hurt again and asked him why he did that. He said he felt like he wasn't really talking to her and that many other people liked her photos too. After him getting angry and several hours of talking, he agreed that he wouldn't contact her at all of any kind over facebook. I have access to all of his accounts and his phone. He admitted to me in the past that he would delete calls and texts from other girls because he didn't want me to get jealous or fight. Now, I have this fear that he is talking to her but is just deleting everything. Technically, he has been doing everything we have agreed upon but it hurts me that he thinks he has done nothing wrong. And it hurts me that he doesn't even seem remorseful. We have talked about this a lot for the past couple of weeks but we get nowhere. Today he finally said that he is just sick and tired of talking about this and that he is done with this convo. Am I just being crazy and irrational? Am I hanging onto nothing here? Do I have anything to worry about? Am I causing problems when I don't need to be? Should I just stop obsessing and just get over it? I really don't want to give into him and then to have this happen again. Im trying to stand up for myself but it's like talking to a brick wall with him sometimes. Please let me know what you think. Like him, I want to get past this too....But for some reason I can't.

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