How should I react to this coworker? Too into me? Or ashamed of me?

[C Treasures]

me and this coworker, we flirt, I mean FLIRT! It's known that I'm in a long time relationship, he always liked liked me from the beginning, but always said he wouldn't want to come and mess something good. But then, as we became friends on social media sites and at work, he realized that I have issues in my relationship. He became more persistent, and I never fully fell for it, although I like him a lot. Most women like him actually. But anyway, a long story short, the real question I have, he would be mean to me, then become nice again and I would just accept him. But what I don't get is how comfortable he is with other females at work, publicly, but when it comes to me, it's always awkward, publicly. He would be playful with other females, but me, it's almost like he is afraid of making any contact with me. But behind close doors, he is all over me and usually turn on and want to touch me and more.... I feel some kind of way that he is not as comfortable with me as others. I'm not sure if he is somehow uncomfortable to be seen with me (no reason for that. The only thing is, I am from a foreign poor country but I'm gorgeous, I'm educated and smart but with an accent)? Or is true that some men are never comfortable around the woman they really like? He is bold with me, in private, not afraid to speak, in general and would send me pictures of himself out of nowhere, but just be cool around me, never. How should I take this?