He has a girlfriend but he really likes me?

So I've known this guy for around a month. We work together. Started off as friends, chatting occasionally, but that very quickly turned into flirting. Honestly, he's really attractive. He kept saying how much he likes me and how he wants to spend time with me etc. Then one day we met up on a day off, and things escalated very quickly. Before I knew it, we were having sex. It felt great and I was so happy (so was he). We spend the rest of the day together at the beach and we chatted for hours about life. It felt so nice having someone who was taking an interest in me. But then... I got home and we were on the phone, and he told me he wanted to take things slowly because HE HAS A GIRLFRIEND. I'm not a slut/whore, so if I'd known this from the start there is no way in hell I would have kissed him or had sex with him. I felt so sick and anxious. And now I'm in such a massive dilemma because he keeps telling me he likes me a lot and finds me attractive and that he wants to continue our "relationship". He also said that he is considering breaking up with his girlfriend (although this hasn't happened...) I just don't know what to do. Aside from the fact he is taken, I really do like him. If he was single I would definitely be trying to have a relationship with him. But he has a girlfriend who he is very indecisive about breaking up with. Need advice please! And try to keep things nice... I'm such a self conscious and anxious mess right now...
He has a girlfriend but he really likes me?
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