I get it. I'm actually glad my ex is married ( not an OW - he met her about a year after he left) as I hope she'll care for my ex in his old age so my kids don't have to.
And although it might look on the surface like he's got things better - young wife, bigger income - life has thrown him some curveballs in the last few years. Meanwhile I have the better relationship with the kids, better health, and financial stability.
In fact, I was discussing with one son today the mechanics of an impending move of another son in college. I am not in town one weekend at that time, so at first I thought "No problem, your dad can drive the rental truck and help you move". Except I realized, he's had shoulder surgery and apparently his shoulder is still very impaired. So he wouldn't be able to lift boxes and furniture and possibly couldn't even drive the truck,
Meanwhile even though I'm 4 years older, at 62 I can still move boxes and furniture and I'm pretty good at driving a UHaul truck. So we'll need to schedule the move for a weekend I'm available. (You might ask why my sons are driving the truck but for a variety of reasons none of them are good choices to drive it.)
