I agree with much of what's already been said.

I've put up with a lot of things from my DP that I wasn't happy about. Not the same as your situation but bad temper, emotional manipulation and various other things where nothing is big enough to walk away but there is enough to really weaken the relationship and cause a lot of resentment.

DP has 'got away with it' for a very long time because I didn't set a boundary - I didn't say 'if you keep treating me like this, I will walk away.' And I should have done.

I know from experience that it's really tempting when the other person says 'let's move on' to think, 'yes, that sounds sensible' - why keep going over the same old ground. DP is a master at saying 'I can't change the past, I can only work on the future'. Sounds appealing doesn't it? But the problem is that the actions of the past cause hurt and resentment.

As to what to do - well yes, that's entirely up to you. I haven't said to my DP - 'if you talk to me like that again, I'm leaving' because I think that's too big a call but I have made it very clear to him that what he's done has seriously damaged the relationship and there needs to be change if I'm going to stick with it.

He needs to work out why he was flirting/making arrangements in the first place, what made him feel he wanted to do that and how can he avoid doing it again. Saying, "I won't do it again," doesn't really cut it and wouldn't give me much reassurance.