Congratulations on getting the commitment to go to Retrouvaille. That is a huge step!
My advice is to just keep the boat afloat til then. Don't try to fix anything. Be nice to him. He should be nice back. An apology is a very good step. You could thank him for the apology. Just keep everything nice. Try to put off all arguments til after Retrouvaille. (Somehow they go away by themselves after that.) And definitely put off relationship talks. You will have a lot of time at Retrouvaille for that. (And sex too.)
The important thing is to get him to walk in their door. They will take it from there. All you have to do is follow the rules and fill your notebook with the truth. They will tell him he has to get rid of the other woman. They will tell you that they want you to go with an open mind and a willing heart. That scared the crap out of PonyGirl's H and he tried to back out. But she got him there.
I know it is hard to wait. I remember feeling it was such an emergency while we waited. But the time will pass, and things will get much better.
