I have feeling for my work colleague and unsure of how he feels

Me and my male colleague shared a kiss at the works party last year. I am in a relationship, so nothing ever became of it! About 5 months ago, other colleagues commented on our behaviour towards each other, it naturally progressed to texts messages. Innocent at first, but as the weeks/months went out they became more intense and pointed out that we both wanted more. I am still in the relationship but haven't been happy for at least a year, this is the reason me and the colleague never went beyond texts. However knowing that we both wanted more we planned on moving things on. We met at work and shared a few kisses which we both agreed made us want each other more! However, out of the blue, my colleague has backed away completely, leaving me slightly confused. Texts have grounded to a halt and he gives me short answers and direct to the point, not flirty like they used to be! He says he feels guilty about my botfriend, although we have discussed this may times over the 4 months and it was never a problem! The only thing I done was drunkly sent him a message telling him I perhaps liked him more than I should given my situation! But he said this had no impact on his choice to back off! He even went on holiday and text me the moment he got home, which makes me wonder, is his guilt perhaps down to the fact that my feelings aren't a one way thing!? Could he be feeling guilty because he has feelings for me? My friends think he cares, why else would we talk the way we do and meet a work for kisses!? I am so confused! I'm not one for showing my feelings, so for openly admitting that I do care for him, is a big deal to me! It makes seeing him at work awkward and unsure of how to act around him! He wanted these things just as much as me - he would write me notes on where and when to meet him for kisses and then tell me how much he wanted me! I'm confused! I am in a relationship, I know it's wrong, but I have been so unhappy for such a long time - I liked this guy at work for years but always too scared to make a move... Now I feel like an idiot for falling for him like I have! I just need someone to help me understand what has made him change, literally, within one day! Does he care for me, but just too scared to admit it?