On top of that ^^^^ why in gods name would you PRETEND to be just fine with him hanging out one-on-one and doing date like activities with someone he basically cheated on you with?
You have successfully set yourself up to be at the very least, totally disrespected and at the worst you've practically given him permission to cheat on you with her, once again.
You really need to stop framing him as the prize, and consider yourself the prize instead and if he can't stop dating other women while with you, then he doesn't get to have you. Stop being so desperate for a wanker that you'd lie down and let him torture you like you're doing. You obviously don't like what he's doing so why would you pretend that you're fine with it? That's sad and he will take advantage of your fear. If not with her (if he's not already doing her) then with some other woman in the near future.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion