Welcome back. You and I are in exactly the same boat except my D is already finalized. Same friend issue, same patterns same staying over each other's houses. I have the bonus of my two kids (S7 and S3) being around him too and telling me about their activities.
Wish I had some helpful advice for you, but I don't. You are perfrectly right that her seeing someone else is no longer your concern and your are perfectly correct in that it bothers you. Bothers me too. Look out for yourself and find strategies to pull yourself up when you get down about it.
