my problem is that he hasn’t gone on the defensive of minimised it. He’s said it was really stupid and embarrassing and that he would be devastated if I’d done the same.
He doesn't have to go on the defensive, does he? He apologised and admitted what a naughty boy he's been and then just carried on as before. You haven't told him to do anything different. If you do, that's when he will go on the defensive and say you're just jealous over nothing or he'll go on the attack and tell you its all your fault and you're a bitch who holds him back.
was saying how much he missed her and how much he liked her, and how he couldn’t wait to see her
Yes that's hanky-panky in itself. They both fancy each other, you heard that. And they don't mind telling each other so while you're in the house. Your DH really doesn't give a toss about your feelings. Maybe it was a snog rather than a shag when she was staying at your house - she'd have to be pretty brassy to go for a shag with you in the house - but your husband thinks he can get away with anything after you let her stay.
The photo was something to do with her visiting a city related to their work project or something
So he hasn't shown it to you. He is lying.
So now you set boundaries. She does not come to the house again, ever. (If she's "stranded" someone else can take her home or she can pony up for a taxi.) He does not call her. Ever. And yes he changes job, well away from a woman he has adoring conversations with when he's drunk. Because that's what he needs to do to save his marriage.
He will not be apologetic once he realises saying "sorry" doesn't hand him a free pass.
it would be professionally impossible for my DH to refuse to go to meetings or dinners if she was there
Well he's got it made then hasn't he? He doesn't need to be defensive because he knows there is nothing you can do.
I also just don't want this to make me someone im not
The problem is not who you are. You're a nice trusting person. It's who he is. Someone who walks all over a nice trusting person.
I don't want to start setting rules and dictating who he's allowed to work with etc.
Well no you don't want to, but you don't have much choice, because if you don't he can just carry on then can't he? And he knows it.
I don't think it's an LTB situation. It's a set boundaries situation. Then you can see how he reacts to the boundaries. But maybe you already guess how he'll react to the boundaries and it's not the way a good husband who'd had a momentary lapse would react.