****This is going to get difficult....What should I do????****?

I will TRY and keep this short and brief even though there is a lot to tell. Me and my coworker have worked together for 2 years through the workplace. Long story short, we are both 28, he became separated from his ex-wife last July and has a 2 year old son. He flirted with me a lot and one thing led to another after a night of drinking. Unfortunately. I know I should have never gotten involved with him from the beginning, I am stupid for that I know. But I am struggling with myself and the choices that I have made as we speak. We have spent almost every other weekend (since he has his son on the other weekends) since January. I guess you could say we were FWB, but a little closer since we sort of already had a friendship at work and were coworkers so we had to keep it somewhat cordual. I noticed a few weeks ago that things started to fade, texts weren't sent as much, and no I am not that pysco type that constantly texts guys. No need to. I won't get into details because this could go into a novel. lol...but a new girl recently started within our group two months ago..we were at a conference about 2 weeks ago and having lunch. Her phone kept going off and she basically had it glued to her the whole time. I actually got a glimpse of her phone as it lit up and it was his name on the phone. Five times during our lunch her phone went off. My blood was boiling at that point and I just couldn't believe it. He is a very outgoing guy, but he flirts with a lot of girls. And yes I know that was actually him because my other coworker at the same conference asked her to text him to ask him a quick related work question, she pulled her phone out and it was the same text she had written seconds before which I did see the last sentence said "hope you have a great day :)" ...I'm not stupid. The day before he told me by text he didn't think having "sex" anymore was a good idea. Saying that he respected me, he's too busy being a dad and a party guy (which I find funny b/c he had no problems before, not saying he has to party, but come on now. And I do understand that being a dad is priority) but just seemed he was beating around the bush and couldn't say I'm not interested. Ok fine. I get it. But it was obvious. Now I'm just angry, because I know that he's been talking to this new girl and just had this gut feeling but after seeing them text back and forth all day at a work conference sickens me to my stomach. I told him that I didn't want to talk to him for awhile and he asked me if I was serious. I said yes. I haven't talked to him since. For two weeks now. At work it's awkward because he walks by my cube real fast. He hasn't been on FB or his IM it's like he's hiding. He did confront me at work when we were alone saying that we both went into this mutually and we knew what we were doing. And that he wants to maintain a civil work relationship and possibly a friendship. That he likes hanging out with me (as he looked away, which I find hard to believe) and I didn't know what to say so I said I don't want to talk right now. He said ok I will leave you alone and he walked away. We haven't talked since. The fact that he broke things off doesn't bother me, I'm mature enough to handle it. But it angers me that I just know that those two have something going on and it was probably going on when I was seeing him. I know we weren't an item, but dam people have feelings. My question is, should I continue to not text him or contact him and stay the distance at work as I have been doing, or just let him know how I feel. I really think he would care less anyways, but who really knows. I have friends that say don't waste your time it's not worth it, and others who say something needs to be said. So far I've said nothing. I just hate that I know and he is probably clueless thinking oh just a bitter chick that is mad I ended things, which isn't the case. And if I do say something I don't feel it's appropriate to tell him oh hey I know your talking to and texting whats her face at work. What do you think I should do? So much childish drama I know, but this is definitely something that is eating away at me. Thanks SO much!!!! :)