From friends to lovers/fighters and back to friends?

Hiyall

Hey, I was friends with this guy I know from work for about 2 years. We eventually began a sexual relationship which I thought would turn into something more. It didn't - he was actually seeing another woman the whole time.

Well I took a couple of weeks off work feeling depressed and angry. But when I saw him again I just wanted everything to go back to the way it used to be. We talked about it and he apologised.

We've been working together "just as friends" for about a month now and he continues to flirt with me. I go from being friendly and playful with him to having flashes of anger for all the pain he put me through, but I try to maintain a civil relationship. It's hard to reject his advances without totally ending the relationship - he is a flirt by nature.

What to do??