I sure feel for you, Rocked. My H's EA partner lived in the city where he most often stayed overnights for business. After he ended things with her he didn't need to be there for a while, but when the first overnight trip came up, I was so stressed out that I had constant headaches from the tension in my neck.
I don't know whether you're doing counselling; if you are, that would be a good place to bring the topic of business trips up with him, and how he can reassure you during them.
Definitely do the GAL and do fun things with the kids, and practice your detachment--continue to remind yourself that you now know you can be fine without him.
At the same time, though, Piecing means fixing any communication problems you had in your marriage, and learning to bring up anything that's really bothering you. You don't want to bludgeon him with recriminations that will "discourage" him, but you don't want to carry a burden alone either (or protect him in a "mothering" way). I chose a time when my H was feeling connected and calm, and gently explained that for a long time I would have bad associations when he was overnight in that city, and asked him to call me and check in with me frequently while he was there. Even the fact that I could now tell him that, and he could listen to it, was a huge relief.
I also think it's better that you can't go with him--it would be very painful for you to be in that city, considering its associations, which would put up some walls between you. Also, you don't want to feel that you always have to police him--you need to have those episodes where he proves himself to be "clean" so that you can build up your trust.
Plan to do things that will distract you, without adding further stress, while he's away.
