Oh OP I hope you're ok this morning
what a dick.
I just want to add to the chorus of posters saying that this is all about him, not you. He texts another woman because he likes the attention, the excitement of the 'chase'. He gaslights you so that he gets to carry on feeling superior, desired and in control.
Now you've broken his internal narrative where he plays the put-upon, unhappy partner of a difficult woman, someone who deserves the 'innocent' pastime of texting a 'friend'.
You only see what's really going on when reality hits his story. He deletes messages because he knows it's out of order, and lies to your face to make you doubt yourself when you get suspicious.
And now you've stood up and said 'no'. He'll be surprised - as if a character in a book turned around and started talking back to him. It fits into the narrative he's got going with 'Stace' beautifully, which is why he texted her, but as pp say she's not really interested in him.
He's going to do everything he can do to try and get back to the place he was before, and that means trying to manipulate your emotions to take him back. Please stay strong.