Hi, just to be clear did he tell you this on a day he knew would be hard for you?
I ask because when I read back on the thread I wondered if perhaps he told you for attention?
I know how awful that sounds but it happens.
Is it possible that he didn’t like the fact that your attention was not on him and that’s why he told you?
I agree with the fact that he even thought about meeting this woman is bad enough , but also the timing, given what you were / are going through, speaks volumes.
I really think having some time apart will be good for both of you. Have a discussion with him, calmly, tell him that you need time to think about what’s happened.
Ask yourself if your trust in him has been damaged and if the relationship can be saved, if that’s what you want.
I believe with some distance and time, you will come to the decision that is right for you.