CrazyP puts it very well.
Good luck for your discussion this evening. You have mentioned your DM is staying with you so hopefully she will be a very useful support whatever comes out of your discussion.
Of course, we're all strangers and only you really know yourself, your DH and your marriage, but I can't help thinking that it might strengthen your position if you conveyed to your DH that you're no longer willing to do the 'pick me' dance and you will let him go and move on if he can't properly commit to you as a wife (i.e. independently of your DC).
I think you may have given him the impression that you want to hold on to the marriage at all costs. On that basis, he holds all the power and has no incentive to commit to you emotionally because he has no reason to fear losing you. It's an invitation to take you completely for granted and treat you like a doormat.
And yes, of course, conveying that you are willing to let him go may indeed result in him going. But surely that is better than fighting to keep him against his will? He may start to appreciate you more if he thinks he might actually lose you.
Based on what you've said before, you were about 21 when you got together. I'm guessing he was a similar age. That's not desperately young but it is still pretty young to settle down for life. He may at some level feel that he made a life-time commitment too soon and be regretting not having experienced other relationships before settling down and getting married. I may be way off beam here, but I'm struck by the fact that you were together for 7 years before getting married and your DH seems to have started an intense EA almost as soon as you actually got married.
If this is the case, it is in no way a reflection on you but it may mean that, as CrazyP says, if it isn't this woman, it will be someone else in due course because he is not emotionally equal to the commitment he has made to you.
Anyway, just some thoughts. Thinking of you, OP, and good luck for your discussions this evening.