How do I tell him I can't do this anymore?

Try to imagine what his GF feels or would feel if she knew. He supposedly cares for her, but he would put her through that pain. Is that the kind of person you want to be with? Would you still feel the same way if he did that to you? Because it is very likely that he would cheat on you after a couple of months. Think about all the pain he would put you in. Think about how you were and would be again the woman who made someone feel like their heart was being smashed repeatedly against a brick wall. If you still feel it is worth it after that I don't know what to say.