Not sure I actually had compersion but I did want her to be happy. And that was lost by her actions, going through the motions type thing you've outlined.
What happened for me is I started caring less....and less. I really could stand the concept of " our date nights " being an obligation to me or the marriage. If your heart and soul aren't in it then why bother. I don't want anyone to be with because they think they have to be. And a scheduled date night came in response to dates with bf....not prior.
Getting him excited about marrying you while he's in nre with her seems like a tall order. You might have to just mourn that loss. I think you might be wise not to make it legal until you see how this all plays out. Dates are one thing but distracted during the wedding or reception a whole new level of problem.
Does his best man know of your situation? Might impact bachelor party.