how recent is all this? Understandably, you sound very raw, and my heart goes out to you.
If, for example, he were to end it with this woman tomorrow, would you think you'd be able to put it all behind you and move past it? If so, then i urge you not to panic.
Not only will you lose out, it'll affect you, and yr dd badly.
Our priority here is you two.
What, out of interest, did you end up saying to this woman? Do you feel she is 'playing for keeps'?
Perhaps if dh has said he will end it, but gently, then a time scale needs to be set.
Have you asked him if he wants to leave? Doesn't sound like it, if he won't consider divorce..
If he's not planning to leave you, and there's a chance you can all get past this, it's well worth you 'playing the long game' and gently assist him meet the goals of him ending it gently. It'll not be easy on you, it'll be excrutiating, but if it's what you want, to fight for yr man, then you have to dig in, and dig deep.
If you agree a timeframe, and he doesn't stick to it, just getting my claws out and thinking, as she's 'only' a trainee, what her employer would think of her behaviour? I rather think it may call into question her professional conduct? She has to have conducted this affair with yr dh, within the jobs framework??
What would his family think of all this? If you need some space to think, and some help, could they be called upon?
While you are in that house, even if you do go and spend some time with his parents, you kind of do still have the upper hand.
If you toss him out, you know full well into whose arms you will drive him. Getting him back could be really difficult then.
Main thing sweety? Don't panic. Take a breath and ask what he's going to do now to put this mess he's made right.
Good luck hon, thinking of you!