How do you act with a "co-worker" you no longer have a relationship with?

I work with a co-worker where we tried to have a relationship. I would call it more hanging out for a month. He decided he wanted to see other people so it ended then. I still have feelings for him, and am hurt by this. I don't think I have a chance of getting him back, but I don't think I need to be his good friend right now either.The first day I saw him, I think he felt guilty, so he sucked up to me, but by the second day I believe I was being to friendly after the recent developments, and I still like him. It is all very awkward, from haning out and texting all the time, to nothing? Any suggestions?