I had a lot of the same feelings when I found out my X was taking all these vacations with his MOW. Vacations that were supposed to be mine after sacrificing for and supporting his career for most of my working life. I had to force myself to stop thinking about it and focus on me, on my kids, on anything else. It sucks and I can't do a thing about it so I had to learn to let it go.
Maybe you can make a plan for a future trip, start planning it, even start putting away spare change or a few dollars a week toward it. That way you can shift the focus to your awesome trip whenever they are flaunting their crap.
It's all about detaching, and it takes time but it also takes discipline to change your thinking. Sure, you'll have days when you slip back a bit, but it will get easier to get past it each time. Focus on YOU.