I think it’s clear that they are together because they WANT TO. And they’ve been thru two separations so it’s clear they came back to each other. It’s not as if they never had a chance to be with others.
Charles was with Diana because he HAD TO be even tho he would have preferred to marry Camilla. But marrying someone divorced was against the rules.
And Diana married far too young to have much of a realistic idea of whom she would be compatible with. She just bought into the whole fairy tale aspect, not reality.
William (being older than Harry) was basically his mothers chief confidante after the separation so he grew up fast. And I’m sure he chose his bride well, being determined NOT to follow in his parents footsteps into disaster.
I think their chances of remaining together are pretty good. They didn’t rush into this and they both have some level of maturity; much more so than his parents.
She apparently comes from a functional family with her parents still together so that’s an added plus.