Yes dj, as bug said, you are definitely getting it about detachment. At least, how we talk about it here.
There is no real "more" with detachment. The only thing that might come to mind for an LBS is, "what next?"
There's no "what next" until we're detached. Don't worry about that, for now. Just know that you have already been through "what next" moments... when you met your H the first time and the romance began...
For now... just detach... and it will take time for it to sink in... for you to really be... detached... and you may go through moments of knowing you're detached and then you'll hit another moment and go, "oh... THIS is detached..."
It's a process. You can't force it. Just work on it and let it happen.
Great job on working through things with your kids and especially with your D18.
Please understand that your D18 might "deal with" the OW and become friendly with her. And further understand that's all it's ever going to be. You have been and always will be, her mom.
Being friends with our kids is a great thing, but we are still their parents. They are still our children, until we are gone.
If it works for you, just consider that OW is one of D18's "friends". And really, depending on how old OW is, consider how... odd... it might be for OW to be "partying" and being friends with an 18yr old... it might help you put it into a perspective...
In the end, as I said, great for you to go out and have a fun day with the kids. You certainly showed them you aren't some bump on a log they might have thought you were...