He used me and I am so upset. How can I move on?

I dated a guy I worked with for a few months to discover that he had a girlfriend. I knew something wasnt right the entire time. Everytime I tried to express my feelings about the situation while we were dating but he woould always tell me not to worry, he liked me A LOT. I just knew something wasnt right. In the end he said I was childish, he never wanted to date me because I was crazy and that I needed to get things right in my head before I could ever be with anyone. After he said those things he wouldnt let me say how i felt. Its been a month now and he has started to flirt with me again. I dont reciprocate but it upsets me. How can i move on?